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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
511 PM CST Thursday Nov 27 2014

Update...
issued at 508 PM CST Thursday Nov 27 2014

Many 5 PM temperatures were already at or below minimum forecast
temperatures...and should continue to fall in the clear calm conditions
through middle to late evening. Lows have been lowered to widespread
single digits. Some favored sites may go a bit below zero by middle
evening.

&&

Synopsis...
issued at 345 PM CST Thursday Nov 27 2014

Bright sunny afternoon in progress as Canadian high pressure continues
to build into the County Warning Area from the northwest. However...looks are deceiving
as we are experiencing the coldest Thanksgiving since the 1950s! 3 PM
readings were only in the teens to lower 20s across the County Warning Area and it
was even colder to our northwest...with temperatures still in the
single digits in northern Iowa...Minnesota and eastern Dakotas.

Meanwhile...strong warm air advection to our west was producing
clouds from the Dakotas into western Iowa and into central MO and
were making slow progress to the east.

&&

Short term...(this evening through friday)
issued at 345 PM CST Thursday Nov 27 2014

Forecast focus on temperatures along with a chance of light snow
after midnight.

Tonight...mostly clear skies near sunset...along with calm to
light winds and some snow cover...will allow temperatures to
really plunge quickly. Of particular concern will be portions of
southeast Iowa and western Illinois that received heavier snowfall yesterday.
However...warm air advection clouds will also be progressing east
into our far western County Warning Area early this evening and spreading over all
the County Warning Area by midnight. Winds will also turning southeast overnight
as the high pressure ridge shifts to our east. This will actually
cause temperatures to rise during the night as strong warm air
advection pushes across the County Warning Area. Minimum temperatures in the
single digits and teens will actually occur this evening.

In addition...ECMWF/NAM saturate the column mainly north of Interstate
80 after midnight due to the strong warm air advection/forcing.
Condensation pressure deficits suggest a band of light snow pushing
across the County Warning Area late this evening and overnight...but the better
saturation/forcing is in our north. Will introduce 30-40 probability of precipitation after
midnight and the c-defs suggest a couple of hours of potential accumulating
snow. Because the GFS is dry I did not want to go likely probability of precipitation but will
let the evening shift monitor the situation. For now I believe any
accumulations will be less than one-half inch.

Friday...any lingering light snow or flurries will end early in
the morning in our far northeast counties as the strong warm air
advection shifts into the Great Lakes region. Otherwise we get
into the warm sector with south winds returning warmer temperatures.
Will continue to forecast afternoon temperatures ranging from the lower
30s along Highway 20 to the lower 40s far south...beginning the melting
process all over again. However...the area of southeast Iowa into western Illinois
where the heavier snow fell yesterday will probably be somewhat cooler
compared to surrounding sites.

Long term...(friday night through thursday)
issued at 345 PM CST Thursday Nov 27 2014

Friday night...not much drop in temperatures expected with continued warm
advection. May see some patchy fog with initial moisture
return over cold ground and have left mention from previous shift.
Not sure though if we will see enough visibilities at or below
1-2sm for mention although could be the case over residual snow
covered areas of 2+ inches. It could also end up similar to last
weekend when it was the 2nd night of moisture return right ahead of
Arctic front which ended up with widespread dense fog.

Saturday and Saturday night...Saturday will see warmup peaking as
surface to 850 mb thermal axis moves across County Warning Area... with the challenge
being just how warm to go with complexities of residual snow
cover and any lingering fog/stratus which could become trapped
beneath inversion. 850 mb temperatures are prognosticated in range of 13-16c
over much of County Warning Area and while the bulk of this warming will remain
aloft gusty southwest winds and some mixing generally supportive
of upper 40s to middle 50s. If more solar insolation then could see
more widespread 50s with some areas possibly near 60f south if little
to no snow cover and with deeper mixing near/above 925 mb. Saturday
night...as mentioned this period may Harbor better fog potential of
which could be dense with brunt of moisture advection ahead of Arctic
front. Cant also rule out some drizzle as well. Temperatures remaining mild
with much of the County Warning Area staying in the pre-frontal warm sector most of
the night with the front moving into northwest County Warning Area around daybreak
Sunday am.

Sunday and Sunday night...highs on Sunday will occur during the am
through midday before passage of Arctic front is followed by crashing
temperatures on brisk northwest winds. May see some flurries being wrung out
in Arctic airmass through Sunday night. Temperature drops of 30-40+ degrees
expected between forecast highs on Saturday and forecast lows Sunday
night... which seems fairly common place over past several weeks.
Lows Sunday night anticipated around 10 to 20 degrees with bitter cold
wind chills in single digits above and below zero by 12z Monday am.

Monday and Monday night...Canadian high to maintain cold conditions
through the period with temperatures 15-20+ degrees below normal. Generally
dry as well although flurries may linger Monday. Lows Monday night
in the single digits and teens.

Tuesday through Thursday...upper level trough prognosticated to swing through
the Midwest into midweek maintaining below normal temperatures. Strength of
forcing and magnitude of moisture at this time support limited
measurable precipitation potential and mainly flurries... but this needs to be
watched for any strengthening which would necessitate probability of precipitation. Still
remains considerable uncertainty at and just beyond the scope of the
extended. GFS has been consistent past 4 runs with precipitation signal on
Thursday in stronger moisture return ahead of ejecting rockies trough.
Meanwhile...European model (ecmwf) has been consistently dry but past 4-5 runs show
model varying quite a bit with surface and upper level features. Have
trimmed probability of precipitation but maintained slight chances to account for GFS potential.
Temperatures anticipated to moderate closer to normal late week.

&&

Aviation...(for the 00z tafs through 00z Friday evening)
issued at 508 PM CST Thursday Nov 27 2014

Clear and quiet aviation weather will last through tonight...as
clouds increase...but snow not looks to remain north of eastern
Iowa terminals. Ceilings above 3000 feet are likely overnight and
Friday...with winds becoming southeast abound 10 kts by 10z
Friday.
Ervin

&&

Dvn watches/warnings/advisories...
Iowa...none.
Illinois...none.
MO...none.
&&

$$

Update...Ervin
synopsis...haase
short term...haase
long term...05
aviation...Ervin

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