Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 171124
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
624 AM CDT Thu May 17 2018

.UPDATE...
Issued at 624 AM CDT Thu May 17 2018

Updated to include 12Z aviation discussion below.

&&

.SHORT TERM...(Today and tonight)
Issued at 208 AM CDT Thu May 17 2018

Today will start out mostly sunny with light southeasterly winds.
Temperatures should quickly climb into the lower 80s across southern
Minnesota and the Minnesota River Valley, with cooler temepratures
north of I-94. A weak frontal boundary will set up between these two
temperature regimes, and this will be the focus for a few afternoon
showers and thunderstorms. Confidence is pretty high in the
development of these storms, but coverage will be very scarce so
most locations will remain dry Thursday afternoon. The best chance
for rain is along I-94 west of the Twin Cities.

Early morning satellite imagery together with Rap13 analysis and
surface obs showed a few clouds scattered across the Upper Midwest
with isolated thunderstorms to the west. A cold front was
approaching I-94 from the northeast. Meanwhile southerly winds were
in place south of this boundary.

Today Upper level southwest flow over the western United States will
lead to cyclogenesis across the high plains, and this will lift the
aforementioned cold front boundary to the north as a warm front.
Instability will build south of this boundary, and forecast
soundings show an uncapped atmosphere by 2 or 3 PM this afternoon.
HiRes models are in good agreement in developing convection over
central Minnesota, but for the most part keep Wisconsin dry. For
that reason did a fine-tune adjustment on Pops to target the area
mentioned above.

As alluded to earlier, have high confidence in a few showers and
thunderstorms this afternoon given the synoptic set-up and HiRes
model output. The limiting factor leading to the 20-30 percent pops
is the lack in areal coverage. The threat for severe weather is very
low, almost zero. For starters, the CAPE profile is skinny and thin,
which pretty much limits the hail potential. Cloud bases will be
high, on the order of 1500 to 2000 meters, so that limits the
tornado potential. The only threat left will be some gusty winds
given the inverted V-sounding, but winds throughout the lowest 500mb
of the atmosphere are less than 20 kts, so that limits the severe
wind threat. Precipitation should taper off by sunset, with
overnight lows near 60 in western Minnesota, and near 50 in western
Wisconsin.

.LONG TERM...(Friday through Wednesday)
Issued at 208 AM CDT Thu May 17 2018

Continued the trend of decreasing pops for Sunday. And in fact, the
weekend overall looks drier. Early next week chances for rain will
linger along I-90 and western Wisconsin, but then high pressure will
build over the Great Lakes and keep the Upper Midwest dry for the
middle of the week.

The forcing for precipitation over the Dakotas and northwest
Minnesota is quite impressive with weak upper level ascent, a
surface trough, and low level Fgen in the presence of CAPE. Should
have several rounds of thunderstorms to the west, and these will
encroach on the CWA by Friday evening.

Then a significant change takes place in the fact that the northern
stream wave moves off towards Ontario, while the southern stream wave
becomes more focused across Nebraska and Iowa. As a result,
Minnesota and Wisconsin are left in between where there is little if
any forcing for ascent. Needless to say, precipitation quickly dries
up and this could mean a fairly dry weekend as opposed to the
widespread rain forecast just a short while ago. The southern stream
shortwave trough and frontal boundary still lingers along I-90, so
areas in the southern part of Minnesota and western Wisconsin have
the best chance of rain and thunderstorms on Sunday.

The lack of shear makes for a low threat for severe weather. At this
point, it looks like if there is any threat it would be for heavy
rain. This is not a flash flood set up, but given the very light
winds throughout the column could end up seeing a quick 2-3 inches
over a footprint the size of a quarter to a half a county. This
system slowly shifts off to the east, and cooler/drier weather
returns for the middle of next week.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 624 AM CDT Thu May 17 2018

A trough of low pressure over the high Plains will very slowly
move east through the period. The main impact will be to turn
winds from northeast and east around to the southeast later today
and tonight. Otherwise, warm advection and moisture return could
lead to isolated showers/thunder north of the Minnesota River
valley this afternoon and early evening. However, the better
elevated instability and forcing looks to be mainly north of the
area, and confidence in occurrence is too low to do anything more
than a VCSH mention at this point.

KMSP...Main item of uncertainty is the possibility of SHRA/TSRA
later today. Main time window looks to be from around 20Z to 00Z.
However, most of the CAM guidance suggests activity will be to the
north, so just included VCSH at this point.

/OUTLOOK FOR KMSP/
Friday afternoon...VFR. Southeast wind 5 to 15 kt.
Friday night...VFR. Southeast wind less than 10 kt.
Saturday...MVFR possible with a chance of showers. Southeast wind
around 5 kt becoming northeast.
Saturday night...VFR. Northeast wind 5 to 10 kt becoming east.
Sunday...VFR. East wind 5 to 15 kt.
Sunday night...MVFR possible with a chance of showers and
thunderstorms. East wind 5 to 15 kt becoming northeast.
Monday...MVFR expected with a chance of showers and thunderstorms.
Northeast wind 10 to 20 kt.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
&&

$$

UPDATE...
SHORT TERM...JRB
LONG TERM...JRB
AVIATION...


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