Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 271756
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1156 AM CST Mon Feb 27 2017

.SHORT TERM.../Today through Tonight/
Issued at 346 AM CST Mon Feb 27 2017

Temps, precip trends and fog will all be a concern through the
forecast period. The degree of mixing and radiational cooling north
is causing concerns with the first round of fog this morning.
Although these variances aren`t widespread, Forest City has calm
winds and has dropped all the way down to 5F in fog while
surrounding sites still have some wind with less fog and temps in
the teens or lower 20s. Conditions will vary widely from location to
location through daybreak, but have added some fog wording north and
also south as fog from MO may creep northward into IA on weak
southerly flow and moisture advection.

Any fog should lift this morning leading into sunshine and temp
concerns. Low level winds will not be as favorable for warming as
yesterday, <180 degrees, but a warm advection regime will still be
in place boosting highs above persistence. Confidence is low with
a wide north to south range due to snow pack, but mixing
adjustments because of this would suggest around 40 toward the MN
border up to mid/upper 50s south.

By tonight PoPs will gradually increase as the current short wave
reaching northern CA, and any weaker short waves farther south in
the stronger central CA/southern NV stronger flow, make their way
into the upper and middle MO Valley overnight. This will strengthen
our warm/theta-e advection with much of our forcing and moisture 1-
3km with more spotty moisture and lift 3-6km. This will lead to only
spotty saturation potential looking at soundings. Ice limitations
aloft will lead to patchy light snow/light rain initially but later
into the night the precip seems to be either drizzle or elevated
showers/weak storms as temps surface and aloft warm. Both the NAM
and GFS 295K isent surfaces show a nice baroclinic zone lifting
through which could focus elevated convection with MUCapes several
hundred. Although not as strong as what occurred a few days ago,
estimated effective 1-6km shear is strong enough to suggest a few
stronger, organized cells and possibly small hail. As the northern
CA PV anomaly reaches the Dakotas early Tue morning, the associated
surface reflection will organize and place a warm front across
northern IA. Dewpoints will jump into the 30s along and south of
this feature which may lead to fog development near the front over
the snow pack. The NCEP NMM/ARW, ESRL HRRR and NAM Nest all
highlight this scenario as well so areas of fog wording have been
added overnight.  Do not expect temps to fall much tonight,
especially late, with stratus blossoming early Tue morning with the
increasing low level moisture.

.LONG TERM.../Tuesday through Sunday/
Issued at 346 AM CST Mon Feb 27 2017

On Tuesday, precip will be ongoing in the morning as a deep trough
resides over the Rockies with subtle shortwaves ejecting out of
the base of the trough and a surface low lifts Northeast across
the state. Between 18Z and 00Z the low is progged to move off,
dragging a cold front through the state.

Tuesday night and Wednesday the upper trough shifts into the
Plains and depending on which model verifies precip will be
ongoing during this period as well. The Southeast quarter has the
best chance for a more persistent precip event then further North
and West but periods of light precip will continue through
Wednesday evening when the upper trough finally shifts into the
Great Lakes.

For Thursday Iowa will be in between the Great Lakes trough and a
ridge building over the West and Rockies. While we will cooler,
models also suggest a decent vort max/shortwave dropping through
the flow across Iowa. The question is where across the state will
this go. Euro and Canadian models both peg more of the West and
South while the GFS hits the Northeast. At this point I kept with
persistence and can adjust as hopefully better agreement occurs.

The models suggest another shortwave for Friday but at this point
it should remain North of the state and miss us. Weak ridging then
builds over the state for Saturday but as a little stronger
shortwave comes off the Northern Plains a surface low will develop
and push into the Eastern Dakotas by late day Sunday with a warm
front extending across Iowa. As a minor wave ahead of the main upper
wave passes late Sunday...some precip is possible mainly over the
Eastern half of the state.

The main wave will pass Sunday night through Monday night for
another extended period of light precip and cooler temps. The
warmest days of the period will be Tuesday into Wednesday with
highs around 40 Northwest to the upper 60`s Southeast and next
weekend with 50`s and 60`s common across Iowa. In between we will
see seasonal highs in the 30`s and 40`s.

&&

.AVIATION.../For the 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1152 AM CST Mon Feb 27 2017

Main concerns will be fog potential and thunder chances overnight
into Tuesday. Weak low approaching with favorable convergence
along warm front north for areas IFR fog toward 10-12z. Though
cigs gradually drop through 06z... IFR/LIFR cigs possible aft 12z
over the area. With stronger mid-level forcing will see thunder
chances at KALO south to KDSM/KOTM...though coverage may be
limited enough to wait on introducing this far out. Have left as
-shra for now...with better confidence on timing next 2
 packages. /rev

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Small
LONG TERM...FAB
AVIATION...REV



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