Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
616 PM CDT Monday Oct 20 2014

issued at 330 PM CDT Monday Oct 20 2014

12z upper air analysis has a shortwave moving through the Great Lakes with
an 850mb cold front from Upper Michigan into northern Minnesota.
Satellite trends through middle afternoon shows some diurnal clouds
developing on the western edge of system clouds.

18z surface analysis has a cold front running from Northern Lake Huron
into southeast Missouri. A Post frontal trough ran from khtl into
eastern Iowa. Dew points were in the 30s and 40s across the plains
and western Great Lakes with 50s from the Ohio Valley into the
Southern Plains.


Short term...(this evening through tuesday)
issued at 330 PM CDT Monday Oct 20 2014

Quiet and dry conditions will be seen tonight and Tuesday. The area
will start out with clear skies at sunset. Late tonight flow
develops off Lake Michigan which may induce low clouds across
northeast Illinois that slowly moves west. If this occurs...areas
east of the Mississippi and along/north of I-80 may see low clouds
move back into the area just prior to sunrise.

On Tuesday...any low clouds that move into the eastern half of the
area will stop their westward movement after sunrise. Diurnal
heating during the day will induce mixing to thin out these clouds
while developing some afternoon clouds across the remainder of the
area. Temperatures should average a little below normal with
readings in the 50s. ..08..

Long term...(tuesday night through next monday)
issued at 330 PM CDT Monday Oct 20 2014

Tuesday night through Thursday...12z medium range guidance in good
agreement of Omega upper ridge sliding across the MS River Valley early
this period...with backdoor surface high having influence across the
western Great Lakes and our local area. Cooler/drier surface dewpoints in
decreasing east/northeasterly boundary layer flow could set the
stage for a chilly night Tuesday into Wednesday morning with lows in the low
to middle 30s across the northeastern two thirds of the dvn County Warning Area. More
frost and low lying/valley fog as well. Large diurnal swing under
plentiful insolation on Wednesday with highs in the middle 50s to around 60.
Signals of increasing moist conveyor up to the Lee of approaching
upstream trough complex are there enough to bump up probability of precipitation for showers
across the northwestern third of the County Warning Area by late Wednesday night. Frontal passage and
associated showers to continue to push through the area on Thursday
probably in a decaying fashion. The 12z run NAM and GFS suggest
precipitable water/S increasing to 0.80 to near an inch by Thursday morning...but
probably overdoing low level moisture return. But even undercutting this
saturation and taking into account some decrease in precipitation intensity
as it translate across the County Warning Area Thursday...several areas/mainly west of the
MS river/ still to get from 0.10 to 0.30+ of an inch of rain by late
Thursday afternoon. Clouds and precipitation to hold temperatures in the 50s to near 60
Thursday despite pref-frontal warm air advection surge. Some lingering light rain or
sprinkles in the east Thursday evening...and any clear out with light
convergent surface return flow may Foster some fog by early Friday morning.

Friday through next Monday...longer range indications by ensembles
still paint broad building upper ridge across much of the western 2/3s
of the Continental U.S. By the weekend. Active flow around the periphery of
this feature across Canada to shuttle another vigorous wave down
toward the northwestern Great Lakes Friday into Sat...warm air advection to the south of
this process to increase across the local area Friday with highs by late
afternoon in the middle to upper 60s in much of the area...and that may
be conservative. Potent wave energy in the Pacific northwest to
pummel its way inland this weekend with aid of strong west-southwest
upper jet-let. Mass response of upper ridge get pressed eastward to
allow some of building thermal ridge across the plains to make it across
the local area Sat and especially on Sunday. Weak mixing under
passing low level anticyclone may limit the warm-up potential on Sat in
the upper 60s to middle 70s...but strengthening return flow signal there
on Sunday ahead of approaching l/west trough axis migrating eastward across
the western rockies. Currently prognosticated warm air advection and
marginal mixing support highs Sunday in the middle to upper 70s. The
airmass prognosticated to be arriving by Sunday would warm up into the low
to middle 80s if it where August or early sept. Uncertainty increases
from Sunday night into early next week with timing of upper trough
passage and associated surface front under it. Low level return flow moisture
and instability also in differing views by the 12z run European model (ecmwf)/UKMET
and GFS. The aggressive GFS spawns sctrd showers and thunderstorms
across the County Warning Area ahead of the frontal passage from late Sunday night and Monday...
while the European model (ecmwf) is advertising a dry frontal passage by Monday afternoon. Feel
it will be in between these solutions and for now will go with slight
chance probability of precipitation late Sun night and Monday for now. ..12..


Aviation...(for the 00z tafs through 00z Tuesday evening)
issued at 615 PM CDT Monday Oct 20 2014

VFR conditions will continue through this cycle as a ridge of high
pressure migrates from the northern plains eastward across the
upper Great Lakes. This will cause light winds to veer from the
northwest this evening to east-northeast by midday Tuesday. The
resulting low level northeast flow over the Great Lakes may
produce some low clouds...but at this time...these appear likely
to remain well northeast of the terminals.


Dvn watches/warnings/advisories...


short term...08
long term...12

National Weather Service Glossary of Abbreviations