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fxus63 kdvn 172043 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
342 PM CST Wednesday Mar 17 2010 


Synopsis... 
the last remnants of the widespread stratus and fog from this 
morning was dissipating across the south at 19z. This was in a 
somewhat narrow axis of 40 plus surface dewpoints that ran from southern 
Minnesota through central and southeast Iowa to Tennessee...along the eastern periphery of 
high pressure parked over north central Kansas. Much drier air with 
dewpoints in the teens has been slowly advecting westward from Lake 
Michigan...reaching into the Freeport area this afternoon. Aloft...12z 
analysis indicated an 500 mb ridge extending from Arizona north-NE through the 
northern plains. A broad cyclonic flow was over the southeast Continental U.S....with 
a closed upper low centered just south of Kansas City and another 
over la. Subsidence downstream from the northern plains ridge was 
resulting in sunshine and rapidly warming temperatures where the 
stratus had finally cleared in the forecast area this afternoon. 


&& 


Short term...tonight and Thursday... 
main short term challenges center on potential for fog again 
tonight...and high temperatures Thursday. 


Deep middle level subsidence...between the shortwave ridge to the north 
and departing trough to the south...will be waning after midnight 
tonight as the center of the ridge axis builds south. At the 
surface...the Kansas high is prognosticated to drop south-southeast with its Northern Ridge 
axis riding across the forecast area early Thursday morning. The 
resulting light westerly low level flow and subsidence will likely 
result in another night of weak moisture advection and convergence 
which may saturate the low levels under the inversion...in a setup 
not too dissimilar from the previous two nights. However...today 
there is more widespread afternoon clearing and thus drying of the 
surface and low levels than the past couple days. For now will go 
with areas of fog wording in the south and west in the axis of 
higher afternoon dewpoints where clearing was latest to occur today 
and patchy fog wording over all but the far northeast. With little 
airmass change temperatures should again cool into the 30s north to 
around 40 in the south. 


Thursday is still shaping up to be breezy and mild as middle level flow 
turns more zonal and prefrontal warm advection commences at the 
surface. Forecast soundings support mixing to at least 900 mb... 
which would support MOS guidance highs in the middle 60s and this looks 
reasonable. This deeper mixing and increasing pressure gradient will 
likely bring gusty winds in 12 to 22 knots range by afternoon. 
.Sheets.. 


Long term...Thursday night through next Wednesday... 
still a lot of uncertainty with Friday-Sat storm system. Most of the 
12z models...09z sref...and 7 of 12 12z GFS ensemble members continue 
recent trends of depicting more progressive system. This would 
present colder scenario and thus more snow than rain... but also 
shorten window of precipitation to 24 hours or less with bulk occurring late 
Friday afternoon-Sat am. Still concerns that amplified western ridge 
may result in stronger... digging system that eventually closes off as 
shown by 12z European model (ecmwf) which has been pretty consistent past several 
runs. This would yield heavier quantitative precipitation forecast as well as extend precipitation well into 
Sunday mainly southeast 1/2... with rain going to snow with potential 
for significant accums with fairly substantial deformation zone. 
Given the uncertainties stayed close to HPC... which generally sided 
with blend... adjusting the timing about halfway toward more progressive 
GFS/UK/Gem. 


Thursday night... surface cold front arrives in northwest late. Winds to drop 
off with approaching front and thus sided toward guidance blend 
on mins. 


Friday-Friday night... Friday represents the transition day and represents a very 
challenging forecast with respect to temperatures and precipitation type/amounts. Strong 
elevated warm advection and moisture overrunning surface cold front to 
result in precipitation becoming widespread near 850 mb frontal zone... overtaking 
much of the area by afternoon and Friday night from northwest to southeast. Temperature forecast is 
extremely challenging Friday with potential for big bust depending on 
location of boundary... as likely to be as much as 20-25 degree temperature 
gradient across the frontal zone. In general coolest northwest (40s) to warmest 
southeast (60s)... with temperatures falling most areas in the afternoon. Transition from 
rain to snow still difficult and quite dependent on surface temperatures as couple 
degrees either way makes all difference. If system remains progressive 
and thus colder... then temperatures still too warm northwest third Friday afternoon 
with mix or changeover to snow. For now have kept all precipitation as rain 
during day... but did speed up changeover Friday night. Could see a few inches 
of snow accumulate especially northwest of quadrant cities Friday night. 


Sat-Sat night...precipitation continues during day Sat being forced along 850 mb 
frontal zone... and may shift to the southeast 1/3-1/2 by Sat night depending on 
eventual movement of upper level energy. Thermal parameters support 
snow... possibly mixing with rain far southeast. Warm ground temperatures and surface 
temperatures above freezing will support some melt and make accums difficult... 
though some accumulate possible with potential for moderate to locally 
heavier banded snow given degree of strong baroclinicity and forcing. 
Temperatures only rising few degrees from am lows. 


Sun-Sun night...kept precipitation chances for snow or rain/snow mix southeast half on sun 
given uncertainties with storm system. If more progressive scenario 
pans out then most if not all area will be dry. Trending this direction 
have removed precipitation for Sun night. 


Mon-Wed...area will reside in northwest flow to start with generally 
dry conditions and below normal temperatures. Ensemble mean forecasts of ao/nao 
index down in negative territory through end of month. This likely 
to result in strong thermal gradient with cold air lurking to the north 
and with strong El Nino energized subtropical jet persisting... the 
recipe is favorable for very active/unsettled weather with one or more strong 
systems. The first of which is suggested by medium range late day 6 (tue) 
and day 7 (wed) and have gone with chance probability of precipitation for rain at this time. Likely to 
see cold intrusions in wake of systems and so typical Spring roller 
coaster. ..McClure.. 


&& 


Aviation... 
VFR conditions will prevail through this evening as area remains 
under high pressure. Lingering low level moisture and clear skies 
may again set the stage for late night and early morning fog. 
Conditions will less favorable than the past couple days and at this 
time have potential for MVFR visibilities limited to the Cid and dbq 
terminals. Will continue to monitor trends through evening for 
inclusion of IFR or worse ceilings and visibilities that would be most 
likely in the 08z to 14z timeframe. ..sheets.. 


&& 


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


&& 


$$ 


Sheets/McClure 

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