Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 180520 AAB

Area Forecast Discussion...UPDATED
National Weather Service Twin Cities/Chanhassen MN
1220 AM CDT Tue Jul 18 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 410 PM CDT Mon Jul 17 2017

Robust thunderstorms have developed to the southeast of Fargo this
afternoon, just to the northeast of a surface low to the north of
Aberdeen, South Dakota. Atmosphere out ahead of these storms in MN
is absolutely juiced, with a pool of mid and even upper 70 dewpoints
existing just west of a warm front that was aligned about with I-35
at 4pm.

Where things go from here is a bit uncertain. SPC mesoanalysis page
shows around 3000 j/kg of uncapped mlcape existing along the pool of
mid 70 dewpoints. 850-300mb thickness pattern shows great diffluence
up in central MN, but the thickness lines dive southeast along I-94
from where the current convection is down toward the Twin Cities, so
this supports seeing storms trying to build southeast into the
instability, likely getting a little farther south than what we had
been forecasting. Coordinated with SPC to bring a watch down to St.
Cloud this evening. Don`t know if we we will need to add to the
southeast though, as shear drops off the farther south you get and
we will be well after dark before these storms get here. Biggest
deterrent we have to getting sever weather is mid level lapse rates
are not all that great and we could just end up with loosely
organized multi-cell clusters that produce marginally severe
hail/wind with torrential downpours.

For tonight, there is a lot of spread between both CAMs and
deterministic models with how much activity we see through the night.
We will maintain about a 30 kt southwesterly LLJ through the night
from SW MN up into NW WI. To go with this, we`ll have the cold front
moving across ND/northwest MN dropping into central MN tonight, so
we certainly could see scattered thunderstorms continue through the
night across mainly central MN.

For Tuesday, it is looking wet. However, not seeing a huge severe
threat, with another round of loosely organized widespread multi-
cell clusters. The action may get started pretty early (by mid
morning) though as a wave emanating from the SW monsoon arrives atop
the cold front sagging south. Given the extensive cloud cover and
rounds of precipitation, temperatures will be quite a bit cooler
tomorrow than what we are seeing today. This will limit the amount
of instability we can build up. Given the weak capping expected
along the boundary, we expect fairly widespread showers and
thunderstorms by the afternoon, though with little organization
expected, this should limit the potential for both severe weather
and excessive rainfall (no concentrated heavy rainfall
tracks/training), both threats look much better Wednesday/Wednesday
night, but more on that in the long term.

.LONG TERM...(Tuesday night through Monday)
Issued at 410 PM CDT Mon Jul 17 2017

The active weather pattern remains on track for the remainder of
this week. For that reason, did not make significant changes to the
forecast.  Severe storms and heavy rain are possible Tuesday night,
but the best chance for severe storms appears to be late Wednesday
into Wednesday night. WPC 7day rainfall totals shows a large area of
4 to 6 inches of rain along the I-90 corridor, but a few
point locations could easily see higher amounts.

The overall set up is favorable for storms with a frontal boundary
that will oscillate north and south as a quasi-stationary warm front
that extends east from a surface low that will develop over the
Dakotas. Storms will develop along this boundary, where surface
dewpoints will be in the mid to upper 70, and temps should at least
be in the 80s.

Meanwhile, a zonal 100kt jet will set up along the international
border and this will provide low inertial instability for the
outflow, as well as deep layer shear for organized convection.  The
orientation of this jet will lead to an elevated mixed layer
continually advecting off the high plains, with H700-500 lapse rates
on the order of 7.5 to 8.5 C/km. This will set the stage for very
large CAPE profiles, in particular Wednesday evening.

The combination of the deep layer shear and instability should
provide a significant threat for severe weather. CIPS analogs show a
few wind events along the southern half of MN/WI, which lines up
with the Day3 marginal risk for SPC. In addition, WPC has a
excessive rain slight risk. The pattern continues, through the rest
of the week, with some indication of high pressure and dryer air
possible for the weekend.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1220 AM CDT Tue Jul 18 2017

There are three main concerns for the next 24 hours. The first is
how long current shower and thunderstorm activity stretching from
north of RWF to MSP/RNH/EAU will last tonight. A few models show
it going most of the night, but others taper it off quickly.
Maintained some shower mention for the next couple hours. The
second concern is whether IFR cigs will develop to the north of
the showers overnight into early Tuesday. There are hints of that
between STC and AXN at taf issuance time, but it doesn`t show much
sign of expanding yet. The third concern is when and how widespread
showers and thunderstorms will become late Tuesday morning and
Tuesday afternoon. Models are indicating a band of torrential
rainfall totals from RWF eastward to MSP and EAU.

KMSP...MVFR conditions may persist for a couple more hours before
VFR conditions prevail into Tuesday. There is an outside chance of
IFR cigs overnight, but it appears the best chance will be to the

Wed...VFR. Chc MVFR/TSRA. Winds SE 5-10 kts.
Thu...VFR. Chc MVFR/TSRA late. Winds lgt and vrb.
Fri...VFR. CHc MVFR/TSRA. Winds SE 5-10 kts.




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