Area forecast discussion
National Weather Service quadrant cities Iowa Illinois
331 PM CDT Tuesday may 21 2013
Synopsis...
issued at 240 PM CDT Tuesday may 21 2013
A closed 500 mb low was located over southeastern South Dakota and
southwestern Minnesota early this afternoon. At the surface
elongated low pressure extended from southeast South Dakota into
Wisconsin with two circulation centers located on the western and
eastern portions of the trough. A weak cold front over eastern
Iowa/northwest Illinois this afternoon originated in the Wisconsin
low while a secondary front was just entering northwest Iowa.
Another...stronger...closed 500 mb low was just moving into the
Pacific northwest and should act as a kicker to dislodge the middle
west system.
&&
Short term...(this evening through wednesday)
issued at 240 PM CDT Tuesday may 21 2013
the system over the central U.S. Will stay over Minnesota and
South Dakota tonight and then drop southeast Wednesday keeping the
region in cool cyclonic flow through the period.
Weak convergence along the middle Mississippi Valley cold front and
modest convective available potential energy around 1000 j/kg will provide a chance of some
evening thunderstorms east of the Mississippi River this evening. A
short wave over the Southern Plains this afternoon will lift northeast
tonight spreading more showers into areas along and east of the
Mississippi River late tonight and Wednesday morning. Wednesday
afternoon...as a weakening surface low moves southeast through the
region...wrap around showers will spread over the area with the
better chances along and north of Interstate 80. Cool air
advecting in to the region tonight and Wednesday will bring
temperatures in the 50s and low 60s tonight. Wednesday with cooler
air and place and widespread clouds highs will only be in the 60s
and low 70s.
Long term...(wednesday night through tuesday)
issued at 240 PM CDT Tuesday may 21 2013
Wednesday night and Thursday...showers will look to pivot around
western flank of western Great Lakes trough Wednesday night an may be more
prevalent along and north of i80 from 00z-06z...and then continue
to rotate south and east into Thursday morning. Forecast soundings suggest
deeper saturated soundings Wednesday evening with precipitable water/S pushing over an
inch...but forcing and instability marginal to make the most of those
profiles. Most areas to get 2 tenths of an inch or less by Thursday
morning. Will not mention any thunder Thursday evening with limited
instability profiles. Top-down saturation late Wednesday night into Thursday
morning may trend showers into more of a drizzle regime after 3 am
CDT. A cool damp night with lows in the lower 50s with some upper
40s possible. Secondary satellite vorticity spoke may keep showers
festering across the southeastern half of the forecast area Thursday morning
before that activity moves out or decays by afternoon. Highs held in
the 60s latest 12z run models in good agreement in advertising
seasonably strong backdoor Canadian surface high dumping down the western
Great Lakes late Thursday and Thursday night...with associated cool push and cloud
clear off setting the stage for a chilly night Thursday night. Currently
prognosticated air mass advecting in could support widespread lows by Friday
morning in the lower 40s to upper 30s...except for middle 40s in the
southwestern County Warning Area.
Friday and Saturday...after cool start to the day...Friday looks to be
mainly sunny until some high clouds spill in from the west off
gathering convection in return flow across the plains. Warm air
return flow aloft especially around the 850 mb mb layer could limit
mixing depth and keep Friday highs held in the middle 60s even with a lot
of insolation. Friday night solutions start to diverge between the 12z
GFS...the European model (ecmwf)...UKMET and Gem. The European model (ecmwf) continues to be less
amplified with middle Continental U.S. Upper ridging...allowing mesoscale convective system development
across the Central Plains to Lee of western Continental U.S. Troffiness to ridge-
ride and propagate eastward along 850 mb mb boundary across portions or
all of the dvn County Warning Area later Friday night and into Sat morning. The new 12z
European model (ecmwf) favors heavier elevated convection fed on 850 mb thta-east plume across
the southern half to third of the dvn County Warning Area Friday night..where areas
could get up to an inch of rainfall by Sat morning. This in contrast
to the more amplified ridge and further we axis placement of the
12z GFS...which keeps Friday night and Sat morning basically dry until
elevated precipitation system moves in from the west by late Sat afternoon.
The GFS then suggests low level jet and warm moist conveyor increasing off the
western Gulf will fuel a nocturnal mesoscale convective system type feature Sat night into
Sunday morning across most of the local forecast area. This while the
consistent European model (ecmwf) is further south with low level baroclinic ribbon lay
out as well as southwestern Great Lakes low level ridge and Northeast Boundary
layer push keeping the dvn County Warning Area mainly dry except for some
overrunning showers across the far south Sat night. The Canadian more
similar to the GFS for Sat night and advertises up to 1.5 inches of
rain across The Heart of the County Warning Area by 12z Sunday. With uncertainties...
will have to keep chance probability of precipitation for almost every period which will be an
unfortunate theme that will look to reoccur through the rest of the
long range. Boundary placement...precipitation episodes and outflow/cloud
debris to play havoc with temperature forecast/S through the Holiday weekend.
Sunday through next Tuesday...as stated above...large differences in
middle Continental U.S. Ridge placement of the main axis and amplification...as
well as north-south lay out differences of west-to-east oriented low level
boundary or 850 mb front between the medium range models will average
out to daily chance probability of precipitation though most of this period. The new Euro
continues to be further south with the quasi-stationary low level
thermal ribbon that precipitation will focus on and over and keep most of
the County Warning Area dry until Tuesday...just bathed in seasonably cool northeast
or easterly flow with just the far southern County Warning Area getting clipped by
overrunning showers. This while the GFS and Gem are further north
with the low level convergent axis an break out periodic episodes of
elevated showers or even possible mesoscale convective system type precipitation complexes through
the Holiday weekend over the region. Climatology suggests a
compromise between the two camps of models that the southern third to
half of the dvn County Warning Area will be under the gun for a soggy Holiday
weekend. Below normal temperatures north of any boundary that sets up
along or just south of the County Warning Area...possibly in the low 60s or even 50s
if overrunning rains win out while lake enhanced northeast surface flow
spills across the area especially on Sunday. Differences continue into
Tuesday as the GFS trends drier shifting the mesoscale convective system track to the north across
Minnesota and WI for the middle week period...while the European model (ecmwf) also trends
north but that targets Iowa and northern Illinois for a favored mesoscale convective system track
for the middle week period. ..12..
&&
Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 1208 PM CDT Tuesday may 21 2013
Stalled low pressure over Minnesota will will drop southeast through
eastern Iowa and weaken late tonight and Wednesday morning.
Widespread cumulus based around 35hnd feet this afternoon will
dissipate with loss of heating by early this evening. An area of
showers will lift northeast brushing locations near and east of
the Mississippi River late tonight and Wednesday. At this time it
appears flight conditions will remain VFR in the showers except at
kdbq where ceilings could drop below 3kft. Further west...an area of
MVFR clouds and scattered rain showers will accompany the weakening low
pressure as it moves through the region. MVFR ceilings should reach kcid
after 12z Wednesday.
Dlf
&&
Dvn watches/warnings/advisories...
Iowa...none.
Illinois...none.
MO...none.
&&
$$
Synopsis...dlf
short term...dlf
long term...12
aviation...dlf
