Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 171735
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1235 PM CDT Mon Apr 17 2017

.SHORT TERM...(Today and tonight)
Issued at 337 AM CDT Mon Apr 17 2017

Main fcst concerns in the short term are the exiting sprinkles/-SHRA
this morning and highs/lows today/tonight.

Data analysis at 06z had shortwave energy moving SE across MN/
northern WI. Some moisture increase in westerly flow in the roughly
800-550mb layer ahead of this shortwave was spreading mid clouds
across much of MN/northern IA into western WI early this morning.
The moisture transport/theta-e convergence in this layer along with
some 290-300k warm advection/isentropic lift and the divergence
aloft enough to produce some sprinkles/-SHRA across parts of east-
central to SW MN early this morning, spreading E/SE toward the fcst
area, even as the sfc-800mb layer remains quite dry.

Model runs of 17.00z initialized well. Solutions very similar as
shortwave energy passes this morning then as the next shortwave
coming in from the west moves into NW MN by 12z Tue. short-term fcst
confidence is generally good this cycle.

Moisture/forcing/lift for the sprinkles/-SHRA early this morning
weakens and moves quickly SE of the area in the 12=15z time-frame.
Will continue/linger some -shra/sprinkle mention into the 12-15z
period this morning, but will plan on late adjustments based on
radar/obs trends thru 09-11z. With passage of the mid level trough
axis, mid clouds expected to exit thru the late morning hours
leaving us another mostly sunny afternoon. Some weak 925mb cold
advection spreads across the area this morning, especially the north
half, with model soundings showing shallower diurnal mixing today.
With light NW BL winds today, consensus highs in the mid 50s north
to near 70 south look quite reasonable. Generally clear skies, light
winds and a dry airmass should allow for rather rapid cooling of
temps this evening. Mid/high clouds ahead of the shortwave coming
across the northern plains already start to increase across the
west/north parts of the fcst area later tonight. However, moisture
plume/transport ahead of the low/front and lift with it remain NW of
the fcst area and left the overnight hours tonight dry. Consensus
lows in the mid 30s to mid 40s tonight, coolest along/NE of I-94,
appear well trended.

.LONG TERM...(Tuesday night through Sunday)
Issued at 337 AM CDT Mon Apr 17 2017

For Tuesday thru Wednesday night: main fcst concerns are the -SHRA/
TSRA chances and rain amounts most of the period.

17.00z models in good agreement for the NW MN shortwave trough to
quickly pass N of the area Tue. Break is rather brief, with a
shortwave originating out of the eastern Pac trough to quickly move
into the central plains by late Wed then into the mid/upper MS
valley Wed night. Plenty of model detail differences by Wed/Wed
night, but this not all that unexpected in a busy fast moving
pattern. Fcst confidence is good Tue, then trends average for
Wed/Wed night.

The main/deeper forcing/lift with the passing shortwave Tue moves
across northern MN/WI. Lower level thermodynamic forcing/lift with
the trailing sfc-700mb trough/front still slated to move across the
area Tue. This with PW values climbing to 1 to 1.25 inches in the
moisture plume. Stronger of the moisture transport/theta-e
convergence is north closer to the track of the sfc low/shortwave,
but some theta-e convergence does accompany the sfc-850mb front
across the area. Trend looks to be for a slight slowing of the
frontal passage Tue, allowing perhaps as much as 500 J/KG of MUCAPE
to develop northward into the area ahead of the front for Tue
afternoon into early Tue evening. Left -TSRA probability as slight
chance for now Tue afternoon while extending it into Tue evening over
the SE end of the fcst area. Precip chances Tue mostly 30-50%,
highest north, reasonable for now. Precip amounts Tue looking to
mostly be 1/10 to 2/10 of an inch. Local higher amounts possible,
but no hydro issued expected from the Tue rains.

Next system due into the central plains Wed develops low pressure
into southern IA by 00z Thu with a warm front east across northern
IL. PW values in the airmass to be lifted over the warm front and
north of the low again in the 1 to 1.25 inch range in the inflow
airmass. Deep layered forcing/lift ahead of the sfc low thru mid
level trough spread NE across the area, especially Wed afternoon/
evening. Even with the model detail differences, good consensus for a
broad area of SHRA to spread across mainly the central/south parts
of the fcst area from late Wed morning thru Wed night. Precip
chances in the 70-90% range along/south of I-90 by later Wed
afternoon/night reasonable. MUCAPE over the area progged to remain
rather limited with the warm front looking to extend across southern
IA into northern IL. However, given the strength/depth of the
forcing/lift, continued a slight chance mention of TSRA across
mainly the S half of the fcst area Wed afternoon thru Wed night.
More on rainfall potential and hydrology concerns Wed afternoon/
night in the hydro discussion below. Generally stayed near a blend
of the guidance highs/lows for Tue thru Wed night.

For Thursday thru Sunday (days 4 to 7): main fcst concerns this
period are lingering -SHRA chances Thu then temperatures thru the
weekend.

17.00z medium range models in reasonable agreement for the trough to
pass Thu morning, to be quickly followed by a more northern stream
wave Thu afternoon evening. End result look to be a deformation/
wrap-around band of -RA/-SHRA lingering over some portion of the
fcst area Thu, mainly in the morning. Pattern then looking to turn
quiet for the area Fri thru Sun with the next stronger shortwave and
low progged to track south of the area thru the lower/mid MS valley
Sat/Sun. 17.00z Can-Gem still offers a more northerly track to this
system, but overall trend among models the past 2 to 3 days has been
a gradually southward shift of any day 6/7 system. After a cooler
(cloudy) day Thu, consensus temps trending near the normals for Fri-
Sun reasonable at this time.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1235 PM CDT Mon Apr 17 2017

VFR conditions expected through much if not all of this period.
Light north or northwest winds near or under 10 kts will continue
through the afternoon. Winds will then gradually become
southeasterly overnight ahead of an approaching cold front. By
Tuesday morning south to southeast winds will increase, with gusts
of 20 to 30 kts expected, highest at KRST. VFR ceilings will
gradually lower on Tuesday morning with showers/isolated thunder
expected to increase ahead of the frontal passage late in the
period. Some MVFR ceilings/visibilities are not out of the
question with the frontal passage/showers. Otherwise, as a low-
level jet increases tonight, LLWS may be a concern, but confidence
too low to mention in the TAFs right now.

&&

.HYDROLOGY...Wednesday and Wednesday night
Issued at 337 AM CDT Mon Apr 17 2017

There is the potential for widespread 1 to locally 2+ inches of
rainfall across parts of the area Wed afternoon and night. The focus
at this time would be along and south of I-90. Due to the locally
heavy rains of Fri night, if these rainfall amounts would fall
across parts of NE IA and far SW WI, there would likely be river
flooding concerns. The inflow airmass with this system will be
very moist with PW values around 2 std deviations above normal,
able to support heavy rain producing SHRA/TSRA. There are still
plenty of model detail difference for Wed/Wed night, which have
the potential to impact the chances of heavy rain and where the
heaviest rains may fall. Thus, no flood or flash flood watches
issued at this time. As the details become clearer and if
confidence increases on the placement of any heavy rain threat,
hydrology headlines may yet be needed. Continue to monitor
forecasts closely.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION...JM
HYDROLOGY....RRS



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