Pattern Recognition and Rules of Thumb versus Ensembles The Non-Lake Effect Snow event of 22-23 November 2015

By Richard H. Grumm

National Weather Service State College, PA and

David Zaff National Weather Service, Buffalo, NY

1. Overview

Based on very warm lake (Fig. 1) pattern recognition, lake effect rules of thumb and analogs, Winter Storm Watches and subsequent Lake Effect (LES) winter storm warnings were issued for portions of the Great Lakes downwind of Lake Erie. LES watches and warnings were issued about 33 and 18 hours respectively, prior to the expected onset of the event1. The forecast called for 6 to 12 inches of snow based on LES forecasting concepts using lake surface temperature, 850 hPa temperatures, wind direction, inversion heights, and Cooperative Institute for Precipitation Systems (CIPS: Gravelle et al 2009) analogs (Fig.2) . The forecast discussion supporting the initial watch included:

.. EXPECT LAKE EFFECT SNOW SHOWERS TO INCREASE QUICKLY FIRST OFF LAKE ERIE THEN OFF LAKE ONTARIO DURING THE MORNING SUNDAY. 850 MB TEMPERATURES QUICKLY DROP TO -8 TO -12C ACROSS THE LOWER GREAT LAKES OVER THE COURSE OF THE DAY SUNDAY. THE STRONG COLD AIR ADVECTION THROUGH THE LOWER HALF OF THE ATMOSPHERE WILL PUSH LAKE INDUCED EQUILIBRIUM HEIGHTS TO BETWEEN 10 AND 15 KFT WITH DECENT SATURATION FORECAST IN THE DENDRITIC GROWTH ZONE. FORECAST MODELS CONTINUE TO SHOW FAIRLY LIMITED SYNOPTIC MOISTURE ABOVE ABOUT 700MB... THE MEAN FLOW IN THE LOW-LEVELS TURNS QUICKLY FROM WEST- SOUTHWESTERLY EARLY SUNDAY MORNING TO MAINLY WESTERLY THROUGH THE DAY SUNDAY INTO SUNDAY EVENING... LOCKING IN ON ABOUT 270 TO 280 DEGREES THROUGH SUNDAY NIGHT…AS IS USUALLY THE CASE WITH LAKE EFFECT SNOW... FORECAST SNOW AMOUNTS REMAIN LARGELY UNCERTAIN DEPENDING ON HOW MUCH THE BANDS WIGGLE AROUND IN THE WESTERLY FLOW... BUT GENERALLY 6 TO 12 INCHES IS POSSIBLE IN THE WATCH AREA...

Though not mentioned in the forecast discussion, the CIPS analogs showed 3 events including 18 November 2014 which produced 3 to 5 feet of snow. The 3 analogs that grabbed forecaster’s attention all produced 1 to 5 feet of snow, these events were in the table noted in Fig 2 but not necessarily at the top of the CIPS analog table which was sorted by date not by best analog. Aside from CIPS, the key patterns were based on pre-high resolution model forecast studies and rules of thumb developed in the 1980s and 1990s (Niziol et al 1995 and Reinking, 1993:). The key excerpts for the early Saturday morning area forecast discussions included:

…LAKE EFFECT PRECIPITATION WILL LIKELY BE ONGOING AT THE START OF SUNDAY MORNING. THE BULK OF THE PRECIPITATION WILL BE FALLING IN THE FORM OF SNOW WITH 850 HPA TEMPERATURES SUNDAY MORNING ALREADY DOWN TO -10C WITHIN THE COLD AIR ADVECTION. THE WESTERLY FLOW OVER LAKE ERIE TYPICALLY SUPPORTS MORE BROAD LAKE EFFECT BANDS OF PRECIPITATION. THERE MAY BE A UPSTREAM CONNECTION TO LAKE HURON MIDDAY SUNDAY...WITH SUCH CONNECTION ENHANCING THE SNOWFALL RATES EAST OF LAKE ERIE. WITHOUT SUCH CONNECTION SNOWFALL RATES WILL

1 Based on information from forecasters. This included that “the event was expected to last from about 4AM EST November 22, 2015 through 7AM November 23, 2015”

GENERALLY BE AN INCH OR LESS PER HOUR EAST OF LAKE ERIE...WITH THE HIGHER TERRAIN OF SW NYS SLOWLY EDGING THEIR WAY UPWARDS TO 6 TO 12 INCHES OF SNOW…

These and similar forecast discussions made no mention of model QPF and the probability of snow from ensemble forecast systems. The 16km NCEP SREF which can and does simulate instability based precipitation around the Great Lakes produced very low probabilities of 6.25mm of QPF in and around the regions downwind of Lake Erie (Fig. 3). Six SREF runs are shown of the 12 available. The 0900 UTC (not shown) and 1500 UTC (Fig. 3b) were similar. The SREF forecasts were focused around 6.25 mm of QPF to fall as snow implying around 3 inches of snowfall using a seasonal 12:1 ratio (Baxter et al. 2005)

The NCAR 3km 10-member ensemble issued at 0000 UTC 21 November 2015, like the NCEP SREF showed little threat of a ES event (Fig. 4). The previous forecast cycle, initialized at 0000 UTC 20 November was of too short a forecast range to cover the period of snow. These data show that 2 real-time mesoscale ensembles indicated minimal potential for a significant LES event.

Human forecasters have developed many rules of thumb to overcome the limitation of numerical guidance to include historically poor QPFs during LES events. Many of these rules evolved when model resolutions were on the order 80 to 190km and output from these models came in 12 hour increments. Modern forecast systems on a global scale, including deterministic models and global ensemble forecast systems have horizontal resolutions on the order of 13 to 45km. Regional models have resolutions on the order of 12 to 16km and finer scale models such as the 4km NAM, 3km HRRR, and 3km NCAR ensemble have finer resolutions and are convective allowing. The HRRR has the added advantage of hourly updates, allowing the model to adjust to ongoing weather sensed by observations to include radar.

These finer scale forecast systems often produce extremely accurate and detailed forecasts relative to traditional guidance over the past 20-30 years. The higher resolution convective allowing systems can simulate detailed banding quite well given sufficient forcing. LES is convectively driven and a key forecast parameter is a miminum of ∆13C lake to 850 hPa temperature difference. A study of LES snow variables (Pereira and Muscato 2012) revealed that “only fetch, inversion height, and ice coverage were statistically significant in determining the intensity of the LES event”. Furthermore these 3 parameters accounted for only about 30% of the overall snow variability. The instability is a critical component of LES snow events and the oft cited difference from the lake surface to 850 hPa of ∆13C was important but larger instability was not found to be statistically significant in characterizing LES events. This low correlation implies that parameter and rule-of-thumb based forecasting has serious limitations which may only be overcome with finer resolution ensembles and rapidly updating models.

This paper will show the larger scale pattern associated with the non-LES event of 22-23 November 2015. The focus will be on forecasts from the 16km NCEP SREF and the 3km NCAR 10-member ensemble forecast system (EFS) which forecast a minimal LES event while human forecasters using pattern recognition, rules-of-thumb, and analogs forecasted a significant LES event. The success of the high resolution guidance versus the human forecasts raises questions about future direction in weather forecasting.

2. The pattern

The large scale pattern over the United States showed a strong 500 hPa short-wave with below normal 500 hPa heights moving into the Great Lakes ( Fig 5). This system produced the first significant winter storm with snow across the Midwest into the central Great Lakes (Fig.6) as a deep surface cyclone moved across the region (Fig 7) and cold air with 850 hPa temperatures well below 0C were present on the northern side of the surface cyclone (Fig. 8). A modestly strong easterly low-level jet developed just north of the 850 hPa cyclone (Fig. 9). The pattern in the Midwest was ideal for a snow event from a pattern recognition perspective and as shown in Figure 4, the NCAR ensemble clearly predicted significant snowfall in the central Lakes.

The strong 500 hPa trough, cold air with 850 hPa temperatures of -6 to -8C relative to a 9C lake surface (Fig. 1) indicated good instability over Lake Erie and the850 hPa winds showed westerly winds over the lake implying broad lake effect bands band. Forecast discussions conveyed information about more broad band lake effect bands.

3. SREF Forecasts

The NCEP SREF ensemble mean forecasts showed 850 hPa temperatures in the -8 to 10C range over Lake Erie by 1800 UTC 22 November 2015 (Fig. 10) with generally westerly 850 hPa winds (Fig. 11). Note the strongest 850 hPa winds were near and north of Lake Ontario.

The SREF mean QPF and each members 6.25 mm contour if present for the period were snow was the primary precipitation type (Fig. 3) showed a small area downwind of Lake Erie where 6.25 mm or more QPF would fall. As forecast length decreased the area and the number of members producing 6.25 mm or more QPF decreased. This is clearly seen in Figure 3 which showed the probability of 6.25 mm or more QPF.

The probability of 12.5 mm or more QPF in the SREF (Fig. 12) showed a very low probability of over 12.5 mm. using a 10:1 and 15:1 SLR the chance of over 5 inches was low. There is little indication in the 16km SREF for a significant LES event.

4. NCAR forecasts

The NCAR ensembles initialized at 0000 UTC 21 and 22 November were used here as these two cycles covered the onset time and the latter run from 0000 UTC 22 November 2015 covered what was expected to be the full range of the LES event. This high resolution convective ensemble should outperform the SREF in forecasting mesoscale and convectively driven events.

As shown in Figure 4, the NCAR ensemble showed little significant threat for 6 or 3 inches of snow.

The updated forecasts at 0000 UTC 22 November (Fig. 13) showed an increased risk of 3 inches of snow in southwestern New York and northwestern Pennsylvania. These forecasts showed a 70 to 90% chance of 3 inches or more snow and a 20% chance of 6 or more inches (Fig. 13 lower panel). The 0000 UTC 22 November NCAR ensemble mean forecasts (Fig. 14-upper panel) showed 1 to 3 inches of snow which

may have been more robust than observed. The earlier cycle (Fig. 14-lower panel), which did not cover the same time period, showed about 1 to 2 inches. Nothing close to the 6 to 12 inches forecast in southern New York and 6 to 8 inches forecast in northwestern Pennsylvania.

5. HRRR

The 3km NCEP HRRR is updated hourly and uses radar to estimate where there is potential precipitation, making it an excellent near-term forecast tool. The HRRR QPFs for 6 runs are shown in Figure 15 with the corresponding reflectivity from these same 4 runs. The QPF values supported 3 to 8 inches of snow in southern New York but diminished as the event neared. The simulated radar data (Fig. 16) showed as forecast horizon decreased the bands were less organized.

Later HRRR runs continued the trend toward lower QPF amounts (Fig. 17), related to a weaker event and shorter time window. A big limitation to the HRRR is the short 15 hour forecast window.

6. Summary

A strong cyclone moved into the Great Lakes on 21-22 November 2015. North and west of the surface cyclone the system brought snow to the Midwest and central Great Lakes. The strong cold advection behind the storm was interpreted by forecasters to present the potential for a significant LES event downwind of Lakes Erie and Ontario. Human forecasts using pattern recognition and rules of thumb predicted 4-8 inches of snow in northwestern Pennsylvania and 6-12 inches of snow in southwestern New York. These forecasts represented a significant departure from the snow and QPF forecasts provided by the 16km NCEP SREF and the 3km NCAR ensemble.

The NCEP SREF at 16km is capable of and routinely produces precipitation due to cold air moving over the warm Lakes. Due to resolution issues and the lack of convection in the system, it often under predicts the maximum QPF and thus snowfall in LES snow events. However, the system can does produce realistic patterns and can help with showing the persistence of LES bands2. In this event the SREF forecast a high probability of 0 to 6.25 mm of QPF and a very low probability of 12.5 mm of QPF implying a high probability of 1-3 inches of snow in the LES bands using an SLR of 10:1 to 12:1. The risk of over 6 inches was extremely low in the SREF in 6, 12, and 24 hour increment. An issue with the SREF and may be low confidence in the QPF as historically many LES rules of thumb developed to overcome the lack of good QPF in LES events.

The NCEP SREF in not a convective allowing model and may not be the best tool to predict LES bands. At 16km it fine enough to resolve the lakes which are on the order of 90 miles wide and 300 to 400 km long3which would provide about 19 grid points over the long axis of a 300km lake. Using the 4∆ to 6∆ rule of the length of a feature a model can resolve and forecast, the SREF should be able to capture some of the processes associated with LES and of course it does. Thus, the SREF typically simulates downwind snow and is quite persistent in the more vigorous events. Two recent examples include the 17-19 November 2014 and 6 January 2015 LES events. In this case the downwind snow in the SREF was 2 Based on personal experience and years of case studies of LES snow in the SREF. 3 Lake Erie is 387 km long and 91 km wide and Ontario is 310km long and 85 km long.

limited in duration and intensity implying a shorter lived event and clearly not a lake effect snow warning event. The SREF resolution needs to be improved and more research conducted to evaluate the SREF QPF efficacy in forecast LES. This research might include the value of bias corrected SREF QPF.

The NCAR ensembles initialized at 0000 UTC 21 and 22 November were used here as these two cycles covered the onset time and the latter run from 0000 UTC 22 November 2015 covered what was expected to be the full range of the LES event. The 0000 UTC 20 November NCAR ensembles did not cover the period where the snow was expected to occur, produced no snow, and was not shown. The 0000 UTC 21 November (Fig. 4) cycle falls nicely into a watch time scale for potential heavy snow and the 0000 UTC 22 November forecasts could have been used for fine tuning warnings, such as convectively based precipitation events like LES. This cycle also covered the entire time of the anticipated event. The latter NCAR ensemble showed support for low-end LES event with a 1-3 inch snow fall being the most likely outcome. The issue with high resolution ensembles may be timeliness. They may provide stronger and more reliable signals but may also require shorter fused outlook products due to availability issues.

The 3km NCAR ensemble has potential to improve forecasts of winter storms and LES events. With 3km resolution each ensemble member should have about 100 grid points along the long axis of a 300km lake. And at 300 km long convective elements should be resolvable and simulated by the members. The NCAR Ensemble and the HRRR may be the best short-term forecast tools available to forecast LES events. The convective nature of LES events implies that outside of pattern recognition and clues for downwind snow in the GEFS and SREF, watches and warnings likely would be more accurate using the finest scale ensembles and model guidance to focus the forecast. The biggest limitation of the NCAR ensemble is the 24 hour interval between forecast updates, but the 48 hour range provides high resolution which covers the watch phase well. The HRRR with a 15 hour window is ideally suited for the warning phase of an LES event though unpublished studies suggest the HRRR still has some systematic bias on the location of the LES bands.

The HRRR data here was useful for updating short-fused forecasts. However, the HRRR does not currently provide uncertainty data and the HRRR forecast range is too short for many high impact winter weather events which often span 12-24 hours. The HRRR time window needs to be expanded as soon as practical. For many winter storms and long-duration LES events, the HRRR forecast window is at times too short to cover the entire event.

This case demonstrates that human rules of thumb, conceptual models, and analogs are often not reliable forecast tools. High resolution ensembles and hourly updated convective allowing models are improving to the point where they are likely more useful then rules of thumb and in the future may supplant them.

Thus, there may be limits to conceptual models when not employed with forecast data from high resolution ensembles and models. The following statement relying on patterns and conceptual models:

.. EXPECT LAKE EFFECT SNOW SHOWERS TO INCREASE QUICKLY FIRST OFF LAKE ERIE THEN OFF LAKE ONTARIO DURING THE MORNING SUNDAY. 850 MB TEMPERATURES QUICKLY DROP TO -8 TO -12C ACROSS THE LOWER GREAT LAKES OVER THE

COURSE OF THE DAY SUNDAY. THE STRONG COLD AIR ADVECTION THROUGH THE LOWER HALF OF THE ATMOSPHERE WILL PUSH LAKE INDUCED EQUILIBRIUM HEIGHTS TO BETWEEN 10 AND 15 KFT WITH DECENT SATURATION FORECAST IN THE DENDRITIC GROWTH ZONE. FORECAST MODELS CONTINUE TO SHOW FAIRLY LIMITED SYNOPTIC MOISTURE ABOVE ABOUT 700MB... THE MEAN FLOW IN THE LOW-LEVELS TURNS QUICKLY FROM WEST- SOUTHWESTERLY EARLY SUNDAY MORNING TO MAINLY WESTERLY THROUGH THE DAY SUNDAY INTO SUNDAY EVENING... LOCKING IN ON ABOUT 270 TO 280 DEGREES THROUGH SUNDAY NIGHT…AS IS USUALLY THE CASE WITH LAKE EFFECT SNOW... FORECAST SNOW AMOUNTS REMAIN LARGELY UNCERTAIN DEPENDING ON HOW MUCH THE BANDS WIGGLE AROUND IN THE WESTERLY FLOW... BUT GENERALLY 6 TO 12 INCHES IS POSSIBLE IN THE WATCH AREA...

may be of limited value when not used in conjunction with high resolution guidance. The bold text in the statement, a human judgement and forecast, and imply that the forecasters saw the clues in the model limiting the LES potential but used rules of thumb and experience to overrule them. There is no mention of QPF and SLR values applied to these QPFs to come up with a probabilistic snow range.

This case may demonstrate the issues the forecasting community faces as we move into an era of higher resolution convective allowing models. It some cases the human may actually degrade the forecast by failing to correctly use the guidance.

A retrospective test of the NCAR EFS needs to be run on the 18-22 two big events November in 2014 and the January 2015. This will provide teaching tools and better examples on how to use these data effectively. The HRRR needs to evolved to a longer perhaps 18-24 hour time window at least 4-8 times a day to help with slightly longer range products such as warnings for and shorter fused watch based products4.

It should be noted that several offices have increased LES lead-time presumably based on pattern recognition. Clearly, with the new tools some training and evolution of the forecast process is necessary. As one of the AFDs in the appendices showed, snow forecasts from the SREF and NCAR ensemble were used and indicated 2-4 inch snow ranges. Yet patterns, rules of thumb, and collaborated forecasts lead to forecasts of 2 to 4 times as much snow.

The verifying liquid equivalent precipitation (Fig. 18) and snow fall showed 2 to 8 mm of precipitation over western Pennsylvania and local maximums up to 12 mm in western New York and portions of eastern Ohio. Snowfall values were considerably lower as the temperatures did not support snow during the first 12 hours of the total accumulation periods. The lower panel shows the QPE during the period of accumulating snowfall. Clearly several locations in western New York had 3-4 inches of snow with lower amounts, around 1 inch in northwestern Pennsylvania and 1 to 2 inches in Ohio. This was not a significant precipitation event during the time as snowfall as indicated by the lower panel showing 8 (.31 in) -12 mm (.47 in) of QPE in the most intense area of snowfall and at best 2mm of QPE in a small portion of northwestern Pennsylvania.

7. Acknowledgements

Pete Jung retrieved all WSW and AFD products from CTP and BUF. Charles Ross provided edits and comments.

4 This is based on NWS watch/warning time periods not all weather enterprise time periods.

8. References

Baxter, MA,C.E Graves, J.T. Moore, 2005: A Climatology of Snow-to-liquid ratio for the Contiguouos United States. WAF,20,729-744.

Gravelle, C. M., C. E. Graves, J. P. Gagan, F. H. Glass, and M. S. Evans, 2009: Winter Weather Guidance From Regional Historical Analogs. Preprints, 23rd Conf. on Weather Analysis and Forecasting, Omaha, NE, Amer. Meteor. Soc., JP3.10.

Niziol, T., W. R. Snyder, and J. S. Waldstreicher, 1995: Winter weather forecasting throughout the Eastern United States. Part IV: Lake effect snow. Wea. Forecasting, 10, 61-77.

Pereira, G., and M. Muscato, 2013: Multivariate analysis of lake-effect snowstorms in western New York. J. Operational Meteor., 1 (14), 157−167, doi: http ://dx.doi.org/10.15191/nwajom.2013.0114.

Reinking, R. F., et al., 1993: The Lake Ontario Winter Storms Project. Bull. Amer. Meteor. Soc., 74, 1828-1849

Figure 1. Plot of Lake Erie temperatures ( C) over the course of the year. The black line is for 2015. Values used in text came from the text database. Return to text.

Figure 2. CIPS analogs from 0000 UTC 21 November 2015 NCEP model forecasts. The top 3 events in this time sorted table produced 3-5, 1-4, and 3 feet of snow respectively. Return to text.

Figure 3. The probability of 6.25 mm or more QPF in the 36 hour period ending at 1200 UTC 23 November 2015. Forecasts are from 6 SREF runs from a) 0300 UTC 20 November 2015, b) 1500 UTC 20 November 2015, c) 0300 UTC 21 November 2015, d) 0900 UTC 21 November 2015, f) 1500 UTC 21 November 2015, and f) 2100 UTC 21 November 2015. The probabilities are color coded and the 6.25 mm contour of the ensemble mean is solid black. SREF data were examined using all runs but only 6 runs are shown here. Return to text.

Figure 4. NCAR 3km ensemble initialized at 0000 UTC 21 November 2015 showing the nearest neighbor probabilities of 6 and 3 inches of snow for the entire 48 hour period ending at 000 UTC 23 November 2015. Return to text.

Figure 5. CFSRV2 500 hPa heights and height anomalies in 12 hour increments from 0000 UTC 20 to 1200 UTC 22 November 2015. Return to text.

Figure 6. GOES visual imagery showing the snow over the Midwest and central Great Lakes. Return to text.

Figure 7. As in Figure 5 except showing surface pressure and surface pressure anomalies. Return to text.

Figure 8. As in Figure 7 except for 850 hPa temperatures and anomalies. Return to text.

Figure 9. As in Figure 8 except for 850 hPa winds and u-wind anomalies. Return to text.

Figure 10.. As in Figure 5 except for 850 hPa temperatures and temperature anomalies. Return to text.

Figure 11. As in Figrue 5 except for 850 hPa winds and u-wind anomalies. Return to text.

Figure 12. As in Figure 3 except for SREF probability of 12.5 mm or more QPF for the for the period from 0600 UTC22 November through 1200 UTC 2 2015. Return to text.

Figure 13. As in Figure 4 except initialized at 0000 UTC 22 November 2015 valid from the 000 UTC 22 November 2015 NCAR ensemble showing (upper) the probability if 3 inches and (lower) the probability of 6 inches of snow. Return to text.

Figure 14. Mean snowfall from the NCAR ensembles initialized at 0000 UTC 21 (upper) and 22 November (lower) 2015. Data show the ensemble mean snowfall in inches. Return to text.

Figure 15. HRR forecasts of total QPF (mm) ending at 1800 UTC 22 November 2015 from 6 HRRR runs initialized hourly from a) 0200 UTC to 0600 UTC 22 November 2015. Return to text.

Figure 16. As in Figure 14 except HRRR simulated radar at 1600 UTC. Return to text.

Figure 17. As in Figure 14 except HRRR model initialized from a) 0700 UTC through f) 1200 UTC 22 November 2015. Return to text.

Figure 18. Event total liquid equivalent precipitation (mm) from precipitation onset time from the stage-IV data and event total snow fall from COOP and public information statements in inches. Return to text.

AFD APPENDICES

AFD Appendix

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...(AFD Friday afternoon 20 Nov) A FAST MOVING WAVE AND STRONG COLD FRONT WILL TRIGGER SOME LIGHT RAIN WHICH WILL TURN TO SNOW QUICKLY IN THE NORTHWESTERN AREAS SATURDAY NIGHT. FARTHER EAST VERY LIGHT RAIN LIKELY ENDING AS SNOW SHOWERS OR FLURRIES SUNDAY. IT WILL TURN MARKEDLY COLDER EARLY SUNDAY AND BREEZY. THE COLD AIR...WARM LAKES AND WESTERLY WINDS INDICATE A POTENTIAL ERIE SNOW BAND. AT THIS TIME SREF IS SUGGESTING 2-4 INCHES POSSIBLE NEAR NY BORDER...BUT BEST SNOW IS LIKELY TO THE NORTH OF THE PA/NY BORDER. WE WILL WAIT TO SEE 3KM NCAR ENSEMBLE TONIGHT AND HOW IT FORECASTS THE SNOW. WE SHOWED GENERALLY 1-3 INCHES AT THIS TIME IN THE LES EVENT AND WILL WAIT FOR HIGHER RESOLUTION GUIDANCE. AFD 325 AM Saturday 21 Nov:

THE COLD AIR...WARM LAKES AND MEAN SFC-800 MB 250-270 DEG WINDS

INDICATE A POTENTIAL FOR SINGLE/INTENSE LAKE ERIE SNOW BAND

INITIALLY ALONG THE I-90 CORRIDOR LATER TONIGHT/EARLY SUNDAY

MORNING....TRANSITIONING INTO A MORE MULTI-BAND SETUP DURING THE

DAY SUNDAY/SUNDAY NIGHT. AT THIS TIME...LATEST OPERATIONAL MODELS

AND THE SREF IS SUGGESTING 2-4 INCHES POSSIBLE NEAR THE NY

BORDER WITH WARREN COUNTY...BUT BEST SNOW IS LIKELY JUST TO THE

NORTH OF THE PA/NY BORDER.

00Z SAT...3KM NCAR ENSEMBLE DATA SHOWS A RUNNING SNOW ACCUM TOTAL

OF 2-3 INCHES BY 18Z SUNDAY ACROSS SMALL PORTIONS OF NW WARREN

COUNTY WITH LESS THAN 2 INCHES ELSEWHERE ACROSS WARREN AND MCKEAN

COUNTIES.

A FEW INCHES OF ADDITIONAL SNOW ACCUMULATION ARE POSSIBLE LATER

SUNDAY INTO SUNDAY NIGHT UNDER MEAN WESTERLY FLOW WITH CLOUD TOPS

TEMPS DIPPING INTO THE MORE FAVORABLE DSG ZONE BELOW -13C.

STILL...THE SUBSIDENCE INVERSION BASE IS NOT ALL THAT IMPRESSIVE

FOR THIS TIME OF YEAR /BEING ONLY AROUND 6 KFT AGL/.

WFO BUF CURRENTLY HAS A LES WATCH POSTED FOR LATER TONIGHT

THROUGH MONDAY MORNING FOR THEIR WRN NEW YORK COUNTIES /GIVING

CONSIDERATION TO A MEANDERING SINGLE BAND NEAR INTERSTATE

90...THEN MAINLY MULTIPLE BANDS/.

WE/RE ALSO LEANING TWD A LES WATCH FOR NW WARREN COUNTY AT THIS

TIME - WITH THE POTENTIAL FOR A FEW OF THE PERENNIAL SNOWBELT

LOCATIONS ACROSS FAR NW WARREN COUNTY PICKING UP MINIMAL WARNING

CRITERIA SNOWS OF 5-8 INCHES OVER A 18-24 HOUR PERIOD ENDING AT

12Z MONDAY. THE REST OF WARREN AND MCKEAN COUNTY WOULD LIKELY SEE

BETWEEN 1 AND 3 INCHES DURING THIS SAME PERIOD. THE LES WATCH

WOULD BEGIN AROUND 09Z SUNDAY AND RUN THROUGH 09Z MONDAY.