Forecasting Ceilings in Forecasting Ceilings in EVENTSEVENTS

Meteorology 415Meteorology 415

Fall 2012Fall 2012

Low CeilingsLow Ceilings

Any broken to overcast deck below 1000 ftAny broken to overcast deck below 1000 ft Part of MVFR (3miles and 1,000 ft)Part of MVFR (3miles and 1,000 ft) Processes in the PBL play the major roleProcesses in the PBL play the major role

Elevation is a prime factorElevation is a prime factor Precipitation (recent or current) has an influencePrecipitation (recent or current) has an influence Direction of surface air flow (upslope is a must)Direction of surface air flow (upslope is a must) Fog season climatology is very importantFog season climatology is very important

Low CeilingsLow Ceilings

Expected precipitation is a key to lowering Expected precipitation is a key to lowering ceilingsceilings

Advection of a low cloud deck, especially Advection of a low cloud deck, especially below a frontal inversion in the fallbelow a frontal inversion in the fall

Mechanical mixing plays a role in a moistened Mechanical mixing plays a role in a moistened boundary layerboundary layer

Low Ceilings – Check ListLow Ceilings – Check List

Consider the following when predicting Consider the following when predicting ceilings below 1,000 feet (IFR)ceilings below 1,000 feet (IFR) Is the station at or near sea level?Is the station at or near sea level? Does the station have less than 12” of rain per year Does the station have less than 12” of rain per year

(Is it in the desert)?(Is it in the desert)? Is the station in the Tropics (even south Florida Is the station in the Tropics (even south Florida

counts)?counts)? ALL OF THESE ARE STRIKES AGAINST ALL OF THESE ARE STRIKES AGAINST

FORECASTING LOW CEILINGSFORECASTING LOW CEILINGS

Low CeilingsLow Ceilings

Also check out the RUC and HRRR

GFS LAMP is a very good tool for short range GFS LAMP is a very good tool for short range ceiling forecasts:ceiling forecasts:

http://www.mdl.nws.noaa.gov/~glmp/glmp_expr.php

GFS LAMP - Low CeilingsGFS LAMP - Low Ceilings

Low CeilingsLow Ceilings Consider the T-Tdwp Spread, but also look at the 850 REL (>90%)Consider the T-Tdwp Spread, but also look at the 850 REL (>90%)

Low Ceilings - SummaryLow Ceilings - Summary

Consider advection of low ceilingsConsider advection of low ceilings Decide on whether conditions warrant the Decide on whether conditions warrant the

formation of low ceilingsformation of low ceilings Rapid moistening of low levelsRapid moistening of low levels Winds become lighter or blow upslopeWinds become lighter or blow upslope Surface convergence expectedSurface convergence expected Cooling of lower troposphereCooling of lower troposphere

Or warming in the winter over cold ground / snow coverOr warming in the winter over cold ground / snow cover Remember instrument’s ability to detect low ceilingsRemember instrument’s ability to detect low ceilings

Low CeilingsLow Ceilings

Become familiar with Become familiar with the Weather Depiction the Weather Depiction Chart showing ceilings Chart showing ceilings and low visibilities…and low visibilities…