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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
545 PM CST Sat Feb 28 2015

issued at 301 PM CST Sat Feb 28 2015

Analysis shows a weak disturbance in Central Plains that is a bit overdone
on bl moisture as usual by solutions. Nowcast tools suggest light snow
and flurries south of I-80 and spotty flurries along I-80 until middle
to late evening. Upstream energy shows clouds to hang around next 24
plus hours with next disturbance arriving late PM Sunday that will
be south evening system...possibly entirely south of area.


Short term...(this evening through sunday)
issued at 301 PM CST Sat Feb 28 2015

Short term forecast confidence assessment...good or above average. Risk
is for even lower snow amounts south sections...many if not most places.
Temperatures tonight with clouds many locations may still be a few degrees
too cold...due to limited radiational cooling.

Tonight...raised temperatures a degree or two and lowered probability of precipitation and snow amounts
with dry conditions north of Highway 30...except possibly some isolated
flurries. Trace to a few tenths possible between Highway 30 and the
Highway 34 corridor in Iowa...east to Galesburg. An inch to very isolated
up to near 2 inches may occur along and Memphis to Keokuk to a Macomb
Illinois line. Most locations south of Highway 34 should receive around
a half inch to possibly an inch. Light southeast to south winds.

Sunday...cloudy with possibly light snow with next disturbance mostly
Sunday afternoon south of Highway 34. Best forcing suggest any measurable
snow will be south of the Iowa and Missouri border for later shifts.
Highs upper 20s to around 30 degrees with winds becoming light westerly.

Long term...(sunday night through next saturday)
issued at 301 PM CST Sat Feb 28 2015

Sunday night through Tuesday...with both surface and upper ridging
sliding across the region for a fair weather but seasonably cold Sunday
night and Monday...will focus on challenging low pressure system
that will eject out of the southwestern plains and probably right
across Iowa/far northern Illinois/portions of the dvn County Warning Area by late Tuesday.
Strength of low to middle level ridge and dry air in place suggest that
the new 12z run European model (ecmwf) delaying precipitation arrival at the surface across most
of the area not until after 06z Tuesday may be on to something. But
seasonably strong warm moist conveyor will not be denied flowing up
to the Lee of approaching developing cyclone. Forecast soundings still
suggest precipitable waters to increase to anywhere from 0.60 of an near
an inch from 06z-18z Tuesday depending on the model fed by 40-50+ knots
southwesterly low level jet. This is 170 to near 200 percent of normal for
this time of year. Quantitative precipitation forecast amounts should range from eight
tenths of an inch possible by the time cold air flushes back across
the are from the west as the system departs by Tuesday evening. But
initial lift going into saturation...and what type of
precip/liquid...freezing and frozen/ will that quantitative precipitation forecast fall as is still a
challenging question at this time.

Forecast soundings generally suggest snow with a sleet mix changing over
to freezing rain and sleet south of i80 from midnight through 4-5am
Tuesday when areas along and south of Highway 34 go to all rain with surface
temperatures/wbz rising to at least 32 degrees. Snow and sleet to look to
transition to freezing rain along the i80 corridor from 5 am through
8-9 am when it should change to all rain. Some light ice glaze
possible before the rain transition. North of i80 and especially the
Highway 34 corridor...a longer duration of snow and sleet Tuesday morning
may allow for some wet snow accums of an inch...then switching over
to freezing rain and sleet north of Highway 30 by late morning. Even the
far north may go to all rain by noon through middle afternoon if the
currently prognosticated strong low level warm air advection and moisture increase in southerly
flow verifies. Thus in this type of scenario do not see long
duration of icing and a build up of a thick glaze. But like th
previous shift noted...much of the area will probably need a
headline for a passing wintry mix from late Monday night through midday
Tuesday. Many details still to be hammered out at this point.

Rainfall amounts may approach a half inch in some locations...and
will have to watch for water run off issues over frozen ground
although deeper snow pack will act like a sponge. May have
passing bouts of fog on Tuesday as well where winds go light
convergent around passing low center and if the rain can let up
to allow it to form with the moisture over lingering snow cover. High
temperatures in the low to middle 30s north of Highway 30...middle 30s to around 40 to
the South. Strong west to northwest winds with enhanced incoming
cold conveyor to switch the precipitation back to a rain snow mix to top-
down cooled all wet snow from northwest-to-se as the late afternoon and Tuesday
evening progress. But expect little additional snow accums Tuesday
evening with this process. What may be more of an impact Tuesday night
is strong northwest winds of 20-30 miles per hour and gusts over 35 miles per hour bringing
in Stout cold air advection. With wet surfaces and standing water
from the rain/snow melt earlier in the day...may be a scenario
where there is a quick freeze up for black ice formation which would
impact the Wednesday morning commute.

Wednesday and Thursday...yet another influx of cold Canadian high
pressure looks to be on tap for this period with lows Wednesday night
possibly dipping well below zero north of i80...if enough snow
cover maintains there may be some -5 to -10 readings in these areas
by Thursday morning. A Wind Chill Advisory also possibly needed for Thursday
morning north of i80 if ambient temperatures can get cold enough.

Next Friday and Saturday...after the middle week cold dome exits...
finally some signs that a flattened pattern may allow a moderating
temperature regime across the region into the weekend with little in the way
of organized precipitation evident at this point. ..12..


Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 540 PM CST Sat Feb 28 2015

Light snow with IFR to MVFR conditions will impact Cid...mli and brl
terminals this evening. Flurries may linger overnight into Sunday am
with MVFR to lower VFR conditions. Southeast/S winds 5-10 kts will veer
to southwest Sunday am then westerly Sunday PM while increasing to
10-15 kts with any ceilings lifting to VFR by early PM.


Dvn watches/warnings/advisories...


short term...Nichols
long term...12

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