Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182347
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
647 PM CDT Wed Apr 18 2018

.SHORT TERM.../Tonight through Thursday/
Issued at 317 PM CDT Wed Apr 18 2018

The primary forecast challenge is residual impacts from the late season
winter storm.

Tonight...Forecast Confidence: Medium/High

GOES-16 satellite imagery continues to show a potent upper level system
moving across the upper Midwest. Recent radar mosaic from 3pm this
afternoon continues to depict areas of light to moderate snow rotating
across the CWA from NW to SE.  Visibilities have come up from earlier
in the day...now ranging from 1 to 5 miles in snow. Getting plenty of
reports of moderate to heavy snow across the upper third of Iowa -
ranging from 5 to 10 inches. Highest totals were generally along and
north of US 18. Snow totals were lower than expected further south
/e.g. Waterloo/...mainly due to a pronounced dry slot that moved
through the area...and kept p-type more light freezing rain and drizzle
rather than snow.

Additionally...seeing areas of convective snow showers develop this
afternoon due to steep low level lapse rates and areas of modest
instability /SBCAPE ~ 100 J/kg/ within the PBL.  So although the
strongest synoptic scale forcing and lift has moved east of the CWA,
will continue to see at least occasional snow showers through
this evening. Additional snow accumulations should be rather
light...up to 2 inches in the north and perhaps an inch south.

Based on webcams and road reports...have decided to cancel the Winter
Weather Advisory for the counties along I-80.  Kept the Advisory and
Warning further north for now...due to ongoing snow and continued
partially covered roadways.  However, would not be surprised if this
could be canceled early later this evening.

Otherwise...expecting mostly cloudy and cold conditions overnight
with lows in the 20s.

.LONG TERM.../Thursday night through Wednesday/
Issued at 317 PM CDT Wed Apr 18 2018

Thursday through Wednesday...500 mb heights rise late this week
over the upper midwest as an upper ridge builds across the area.
The next upper low was moving onshore in the Pacific Northwest
this afternoon. It should emerge from the Central Rockies early
this weekend. By that point, it will disconnected from the
westerlies which will be well to the north. As a result, the
models slowly drift the upper low into the Southern Plains late
this weekend. Meanwhile, both the 12Z GFS/ECMWF bring a northern
stream shortwave across the northern plains into the upper midwest
Tuesday/Wednesday which should result in a Pacific cold frontal
passage Tuesday night or Wednesday. With the upper low to the
south, moisture return should be reduced thus lower precipitation
chances look good.

At the surface, a slow moving high pressure system will move across
the region Thursday through the weekend. The sensible weather will
be a several day dry period with moderating temperatures and
relatively low surface winds. Temperatures may exceed normal
temperatures by next Monday which will help dry things out. High
temperature climo is in the lower to middle 60s by early next week.
We should see slightly cooler temperatures but still much warmer
than recent temperatures by the end of the period.

&&

.AVIATION.../For the 00Z TAFS through 00Z Thursday evening/
Issued at 644 PM CDT Wed Apr 18 2018

Precipitation will continue to diminish across the area. Localized
fog is possible overnight after the precipitation but at this time
confidence is low enough to exclude mention. Areas of MVFR to IFR
cigs remain possible overnight then a transition to widespread VFR
on Thursday. The wind will diminish overnight as well and will be
generally from the north at less than 12 kts Thursday.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...

Winter Weather Advisory until 1 AM CDT Thursday for IAZ007-017-
026>028-039.

&&

$$

SHORT TERM...Fowle
LONG TERM...Johnson
AVIATION...Donavon


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