Scientific Forecaster Discussion
Area forecast discussion
National Weather Service quadrant cities Iowa Illinois
953 PM CST Sat Jan 24 2015
issued at 950 PM CST Sat Jan 24 2015
Clipper system still on track to pass across far southern County Warning Area late
tonight. Weaker forcing and dry air far northeast County Warning Area being
suggested and trimmed accums up there. Main forcing and best zone
for precipitation being suggested by latest mesoscale models to set up roughly
in corridor bounded by Independence Iowa to Savanna Illinois north and
Fairfield Iowa to Galesburg Illinois south... or roughly Highway 30 to north
of Highway 34. Soundings support changeover to snow from around 09z
to 15z northwest to southeast in this corridor with several hour
period of light to moderate snow after changeover. Surface temperatures
marginal leading into this and may chew away at snowfall making
for challenging snow amount forecast. Overall still looks like up
to around an inch of accumulate on track this corridor with much of
accums on grassy or elevated sfcs and roadways mainly slushy. Cant
rule out some spots around 2 inches of accumulate with areas mainly
just south of I-80 being favored attim. Temperatures should drop below
freezing by middle to late Sunday am and this will be when some
driving problems may occur with refreeze and possible slick/icy spots
issued at 239 PM CST Sat Jan 24 2015
Analysis at 2 PM shows two lobes of energy diving southeast from Alberta
into western Nebraska. Strongest energy still in Canada clearly shows
system to dig further south with second vorticity maximum to create lots of precipitation
along I-70 corridor Missouri into Illinois to lower forcing with lead
energy center and deformation on north side of surface low over forecast
area. This will lead to lighter snow totals late tonight and through
Sunday middle day. West winds with fair skies allowing for temperatures
to rise well into the 40s to near 50 degrees at 2 PM in west sections.
Short term...(this evening through sunday)
issued at 239 PM CST Sat Jan 24 2015
Short term forecast confidence assessment...fair or average with main
issue risk of localized 3 inch snow totals between Highway 34 and south
of I-80 if second energy center causes a decent deformation zone late
morning. Also question of how quickly rain changes to snow with profiles
suggesting any sustained precipitation should change to snow within
an hour or so due to evaporative cooling. Another issue is strength
of winds and falling temperatures by Saturday afternoon with some gusts
to near 35 miles per hour possible that may allow for minor drifting of snow.
Tonight...skies to become cloudy with light sprinkles or light showers
developing or moving into northwest sections shortly before midnight. Again
any decent precipitation should mix with or change to snow as coverage
increases from northwest to southeast overnight with snow line likely to be south
of Highway 30 by 6 am. Temperatures should be near 32 degrees and combined
with our ground soils above freezing limiting snow accumulations on
roads initially. Indications of a tenth or two of snow along Highway
30 by daybreak to near 1 inch along and north of Highway 20 in Iowa.
Temperatures should remain above freezing for mins along and south
of I-80 corridor.
Sunday...surface low to move south with increasing NE winds of 15 to
30+ miles per hour over all the area by noon. Light to moderate snow of 1 to 3
hours expected most areas as system slides south. Most locations along
and south of Highway 30 to along Highway 34 should receive 1 to 2 inches
with risk of isolated 3 inches possible with second deformation zone
south of I-80 late morning into early afternoon for later shifts to
monitor due to second energy center phasing with first energy center
that arrives late tonight. Highs near 8 am most locations with falling
temperatures into the 20s along with increasing NE winds that may allow
for minor drifting of the wet snow with 10-12 to 1 ratios expected.
Long term...(sunday night through saturday)
issued at 239 PM CST Sat Jan 24 2015
Overview...a longwave trough will develop over the eastern U.S.
Keeping the Midwest in northwest flow aloft through Tuesday. Then a
return to a split upper-level flow is forecast along with temperatures
about 10 f degrees above average in the 30s/40s. A clipper system
may bring light precipitation to the Midwest Wednesday night. Thermal
profiles suggest rain initially possibly changing to show showers
by Thursday morning.
Monday...there will be a large low pressure system developing along
the East Coast. The Midwest will be on the western side of the
longwave middle/upper-level trough. Warm air advection will be ongoing through the day
as 850 mb temperatures warm to near -2 c and 1000-500 mb thicknesses rise
to ~540 dam. Have slight chances for -fzdz with low stratus overhead
and lack of saturation at -10 c or colder. Thinking any light precipitation
would be patchy but will have to monitor. Forecast maximum temperatures range
from middle 40s SW to upper 20s NE.
Maximum temperatures Monday through Wednesday...blended the European model (ecmwf) in with the
consensus blend to essential tighten the maximum T gradient across the
County Warning Area...making it warmer over the SW and cooler over the NE where
stratus will be more stubborn and Snow Field is more prevalent.
Tuesday...impressive middle-level ridge forming over the central U.S.
Upstream of the strong East Coast trough. Warm air advection will continue to
increase 1000-500 mb thickness through the day...reaching near 546
dam by the evening. Current forecast is for near 50 f in the SW dvn
County Warning Area to the middle 30s near Freeport where stratus may hang on longer.
Otherwise...dry conditions expected.
Wednesday...SW to NE temperature gradient across the Midwest looks to peak
on Wednesday. Lower 60s are forecast as close as west Missouri. A shortwave
trough will move into the central/northern plains and resultant surface
low will form downstream and reach west Iowa by the late aftn/evening.
This meteorological setup puts east Iowa and west Illinois in the warm
sector...1000-500 mb thickness may peak as high as 552 dam and 850
temperatures near +6 c within a S/southeast surface wind flow.
There are chances for light precipitation mainly in the form of rain Wednesday
night. Cold air advection on the back side of the surface low should cool the thermal
profile enough for any residual showers to change to light snow.
Thursday and Friday...500 mb flow becomes more zonal as middle-level
vorticity. Maximum dives down toward California/Arizona and another one
rotates southward around the original East Coast low into central
Canada. This may be the fist sign of a significant pattern change to
colder temperatures...possibly commencing next weekend.
For what it is Worth...both the GFS and European model (ecmwf) have 1000-500 mb
thickness values sub-510 dam by next Sun night/Mon. In line with
this raw model output...the CPC outlook for the first week of
February has a ~70% probability for below normal temperatures over east
Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 541 PM CST Sat Jan 24 2015
VFR conditions expected this evening with broken-overcast clouds with
ceilings at or above 6kft above ground level along with sprinkles and scattered light rain
developing middle to late evening ahead of an approaching clipper
system. Conditions will deteriorate to MVFR/IFR overnight and
Sunday am with rain changing to snow (roughly 25/09z-25/15z
from northwest to se)... with period of light to moderate snowfall before
precipitation ends late am through early afternoon as the system moves toward
the Ohio Valley. Winds will be light this evening into the overnight
while backing from west/SW to E/se... then winds will increase from
northeast late tonight and Sunday at 10-20 kts. Conditions should
begin improving to VFR by middle to late Sunday afternoon.