Winter Weather Threat 1/29/2019

Tuesday through Friday...



Record breaking and potentially historic outbreak of cold still

appears to be in the offing for the middle of next week. Greater

than normal degree of forecast uncertainty in temperatures, due in

large part to models showing all time records being broken and in

some cases shattered.



Generally made only very small tweaks to the temperatures from the

model blend, due to the uncertainty. It is worth noting, that this

model blend includes bias-corrected grids which still reflect a

bias correction warmer from the unseasonable warm spell back in

late Dec and early Jan and also contains a MOS component which

tends to have some weighting toward climo, which obviously midweek

temps will not look anything like climo. Typically, in record

breaking temperature situations, a blend of the raw 2m model

temperatures verifies best and those values are colder than our

official forecast and approach or break all time records for ORD

and MDW.



The GFS, GFS-FV3, GEM, and ECMWF 12z operational runs all continue

to drop 850mb temps to below -30C & in the case of the GFS to-

40C. To put this into perspective, using the SPC sounding climo

page, the coldest 850mb temp even observed in IL (ILX, DVN, RAN,

and PIA and dates as far back as 1949) is -32.9C. Over the past

couple days there have been some minor run to run variations in

how low 850mb temps bottom out, but there has been a good deal of

consistency of 850mb at least reach -30C. Even the 51 member ECMWF

ensemble has 850mb temps reaching -30C over northern IL.



Assuming we have a fresh snow pack from Monday`s system, the stage

would be set for temperatures to really get out of hand and end up

colder than currently forecast. Initially, moderate to strong

northwest winds look to send wind chills bottoming out in the -35

to -50F range late Tues night into Wednesday. Some minor

improvement in wind chills is expected Wed night into Thursday

morning, due primarily to diminishing winds. Also worth noting

that the 35 to 50 below wind chills are based on the somewhat

conservative official forecast temps, but should the colder raw 2m

model temps verify we could see wind chills approaching 60 below

in the coldest areas and with most of the CWA reaching close to 50

below.



There is still time for model guidance to back off on the

intensity of Arctic air mass. Keeping that in mind, I felt quite

comfortable with the more conservative temperature forecast. It is

much easier and preferred to slowly trend a forecast closer to

all time records this far out, rather than jumping out forecasting

all time records only to later have to back off on those extremes

if model guidance trends less severe with the cold.



To put this cold into perspective some, during the past 20 years,

Chicago has only had a sub zero high temp twice and only 22 times

in the past 100 years. Our current forecast has air temps below

zero between 40 and 60 hours across most of the CWA, which a much

longer period of sub-zero wind chills.ert quote here