Forecasting Ceilings in EVENTS Meteorology 415 Fall 2012

Low Ceilings Any broken to overcast deck below 1000 ft Any 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 role Processes in the PBL play the major role Elevation is a prime factor Elevation is a prime factor Precipitation (recent or current) has an influence Precipitation (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 important Fog season climatology is very important

Low Ceilings Expected precipitation is a key to lowering ceilings Expected precipitation is a key to lowering ceilings Advection of a low cloud deck, especially below a frontal inversion in the fall Advection of a low cloud deck, especially below a frontal inversion in the fall Mechanical mixing plays a role in a moistened boundary layer Mechanical mixing plays a role in a moistened boundary layer

Low Ceilings – Check List Consider the following when predicting ceilings below 1,000 feet (IFR) Consider the following when predicting 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 (Is it in the desert)? Does the station have less than 12 of rain per year (Is it in the desert)? Is the station in the Tropics (even south Florida counts)? Is the station in the Tropics (even south Florida counts)? ALL OF THESE ARE STRIKES AGAINST FORECASTING LOW CEILINGS ALL OF THESE ARE STRIKES AGAINST FORECASTING LOW CEILINGS

Low Ceilings Also check out the RUC and HRRR

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

GFS LAMP - Low Ceilings GFS LAMP - Low Ceilings

Low 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 - Summary Consider advection of low ceilings Consider advection of low ceilings Decide on whether conditions warrant the formation of low ceilings Decide on whether conditions warrant the formation of low ceilings Rapid moistening of low levels Rapid moistening of low levels Winds become lighter or blow upslope Winds become lighter or blow upslope Surface convergence expected Surface convergence expected Cooling of lower troposphere Cooling of lower troposphere Or warming in the winter over cold ground / snow cover Or warming in the winter over cold ground / snow cover Remember instruments ability to detect low ceilings Remember instruments ability to detect low ceilings

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