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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
640 am CST Thursday Nov 26 2015

issued at 639 am CST Thursday Nov 26 2015

Lightning data shows lightning strikes occurring with the higher
reflectivity showers that developed over S central Iowa over the
past hour. These appear to be occurring along the nose of an 850
mb southwesterly low level jet shown by the VAD wind profile network over
MO and Kansas. This was also in an area of 700 to 500 mb layer lapse
rates in the range of 6.5 to 7 degree c/km shown by the NAM advancing
northeast in the stronger SW flow over Kansas and MO.

Have updated the forecast to include isolated thunderstorms as
this elevated convergence and instability will overspread southeast
Iowa...NE MO into west central Illinois this morning. Also removed mention of
fog along and ahead of the cold front further northwest...where surface
observations this morning have shown the lowest visibilities no
lower than 1 to 3 miles over the past few hours.


issued at 336 am CST Thursday Nov 26 2015

A slow moving surface cold front...aligned under the upper level SW
flow...reached from Kansas northeast bisecting Iowa from SW to NE...into
Upper Michigan. Ahead of the front...southerly winds were advecting a very
mild and moist airmass into the region with temperatures in the 40s
and 50s with even a few sites touching 60 over northern MO at 3 am.
Behind the front...temperatures were in the 20s and 30s. Area
radars showed scattered showers in the warm sector across MO into
Illinois...with precipitation more widely scattered over the local
forecast area outside the far northwest closest to the boundary. Over the
past few hours...increasing low level moisture convergence along
the boundary was leading to fog formation just to the west and northwest
over central and NE Iowa.


Short term...(today and tonight)
issued at 336 am CST Thursday Nov 26 2015

Widespread rain will develop from northwest to southeast today as the cold front
slowly pushes southeast crossing the MS river around 21z to
00z...or 3 PM to 6 PM. Overall...little change will take place in
the forecast...other than minor adjustments to fog timing and
coverage. Still anticipating a significant rainfall event with
totals of 1 to 2 inches over the region.

This morning...precipitation will be mainly in the form of scattered
showers until better forcing aloft arrives to interact with the
advancing frontal zone and baroclinic axis this afternoon and
evening. Have continued the trend of increasing probability of precipitation from northwest to southeast
through the day with quantitative precipitation forecast amounts of less than a quarter inch southeast to
75 to nearly 1 inch in the northwest...where more moderate rain
develops by late morning. Have limited fog coverage to closer to
the boundary in the far northwest early...then do not anticipate
visibilities to be restricted enough to mention during the
daylight hours. South winds from 15 to 25 miles per hour ahead of the front
will continue to advance a very warm and moist airmass
northward...resulting in highs reaching from the middle 50s to
around 60 over the central and east. The earlier frontal passage and onset
of more widespread rain the northwest will limit highs to the middle 40s to
around 50.

Tonight...the period of strongest forcing and feed of 1 to nearly
1.3 inch precipitable water values into the region will produce widespread moderate
rainfall during the evening...which advances east of the area after
midnight with northwest winds and cold air advection to follow. The GFS was
slightly overdone with precipitable water values looking at 00z verification...and
thus its quantitative precipitation forecast was on the high side and appearing to reflect a more
convective nature than the lower values shown by the European model (ecmwf) and NAM.
Going closer to these lower guidance values...our forecast has an
additional .75 to possible 1 inch overnight...with the heavier
values favoring an axis from NE MO across eastern Iowa. Temperatures
will be nearly steady in the 50s in the east early...with falling
temperatures through the rest of the night...reaching the 30s to
around 40 by dawn.

There is still a possibility of mixed precipitation along the back
edge of the system in the cooling airmass. Using a top-down
method...the most likely precipitation type would be sleet or
possibly freezing rain...although whether surface temperatures
will be cold enough to realize ice accumulation is questionable.
For now...will mention a mix of rain...sleet and freezing rain on
the back edge into east central Iowa late night toward morning...but
not carry any accumulations of what would be light wintry
precipitation at best.

Long term...(friday through wednesday)
issued at 336 am CST Thursday Nov 26 2015

Main forecast concern in the long term are is the blocking pattern
over the western United States and subsequent precipitation this
weekend and into early next week. As with previous forecast
discussions...chances for rain during the day...followed by frozen
precipitation at night. There are differences in model solutions for this
weekend....however they converge on a solution for Tuesday.
Regardless the period will be filled with possible changes to the
forecast and even possibly some minor travel concerns.

Friday through Monday morning...

Light to moderate rain will be ongoing across the area. The precipitation
will be on its way out...trying to out run the surge of cold air
associated with a 1038 to 1042....model dependent...surface high
pressure. For the most part....Saturday should be clear of
precipitation. Then this is where the models diverge. The European model (ecmwf)
lumbers a large cutoff vorticity maximum to the east...while the GFS and Gem
are slower. This European model (ecmwf) advects moisture further north sooner than
the other models. As a result...thermal profiles are such that rain
falls during the day with a freezing rain/rain mix at night. Saturday night
there is a schc of this across most of the area and again on Sunday
night. My preference is for the GFS/Gem solution as this makes more
sense from a synoptic scale solution with the cutoff low slower to
move until the next shortwave approaches the West Coast. While I
give a small nod to the European model (ecmwf) confidence is quite low
in the chance freezing rain/rain I have across the area. Regardless...with this should be monitored into other forecasts.

Sunday night into Monday morning...large scale start to phase
between the three models. The European model (ecmwf) is the more bullish of the
three with moisture return to the area. With the current thermal
profiles of the a warm nose advecting into the area...there is once
again a chance for freezing rain and rain. This situation I have slightly more
confidence in than Saturday night. At this time...I dont think
there will be much of any accumulation.

Monday on...

Temperatures warm up enough to change most of the precipitation back to
rain across the area during the day. The models are in agreement
with the large scale flow with a vorticity maximum rotating around the base
of a cutoff low at 500 mb. The placement of this vorticity Mac is across
Kansas and will lead to better forcing and a better chance for
precipitation than anytime during the weekend. Cold air will eventually
move into the area and change the precipitation over to snow Monday night
and into Tuesday. Wrap around from the low could bring some snow to
the area on Tuesday. After this...ridging and high pressure builds
into the area. Models diverge on largescale features...but at this
time all suggest quiet weather.


Aviation...(for the 12z tafs through 12z Friday morning)
issued at 543 am CST Thursday Nov 26 2015

Flight conditions will deteriorate today as a slow moving cold
front spreads low clouds...fog and rain across the terminals.
VFR conditions at sunrise...with scattered showers...will give
way to MVFR conditions by midday. In the afternoon...all sites are
expected to worsen to IFR due to lowering ceilings and visibility
reductions in rain and fog. Winds will veer from S-SW to northwest-north late
afternoon and evening as the front pushes through with widespread
rain and IFR or even LIFR conditions. Conditions will likely
remain IFR through the night with precipitation ending. There is a
low potential that a mix of light freezing rain and sleet may
affect the Cid and dbq terminals toward sunrise Friday
morning...but confidence is too low to include in the forecasts at
this time.


issued at 336 am CST Thursday Nov 26 2015

Long duration rainfall event today through Friday morning still
on track to bring rainfall amounts from 1 to 2 plus inches across
the area. With some snowpack still on the ground and saturated
soils...a large percentage of this steady rain could go straight
into runoff. Ensemble river forecasts suggest many area rivers
making flood stage when the highest quantitative precipitation forecast potential is used. Using
the latest forecast amounts...the Iowa river at Marengo...English
River at Kalona...and north Skunk River at Sigourney make flood
stage by the weekend. Several other rivers or river sections make
action stage...such as the rock...Pecatonica... wapsi...and lower
sections of the main Stem Mississippi from Keithsburg on south by
Sunday or early next week. Will hold off any early issuance of
river flood warnings for now to better see how rainfall and amount
potential will lay out over the next 12-24 hours.


Dvn watches/warnings/advisories...


short term...sheets
long term...Gibbs

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