Area Forecast Discussion
Issued by NWS Twin Cities, MN

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455
FXUS63 KMPX 210812
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
312 AM CDT Mon May 21 2018

.SHORT TERM...(Today and tonight)
Issued at 310 AM CDT Mon May 21 2018

The main challenge in todays forecast is how the interaction of the
upper level disturbance located across South Dakota, and
weak theta-e advection across Iowa/southern Minnesota slowly evolve.

Based on current radar and satellite trends, the bulk of the
activity in southwest Minnesota will lift northward and slowly
dissipate toward noon, while another area developing across northern
Iowa, will spread across south central/southeast Minnesota through
noon. Between these two systems that are generating scattered shower
activity, will be a lull with only isolated showers possible.
Current pops convey this trend with the highest across
southwest/west central Minnesota until noon, and another area in
southeast Minnesota, and portions of west central Wisconsin slowly
increasing in areal coverage through noon. By the afternoon, most of
the activity will slowly begin to dissipate as the main upper level
disturbance moves overhead and east, with any theta-e advection
weakening with time. Temperatures in the 60s will reflect the denser
cloud cover and scattered shower activity. Later tonight, especially
in areas that had more rainfall, fog will likely form as light winds
hold across the area, and skies clear. I wouldn`t be surprised to
see areas of dense fog where skies clear quicker in western
Minnesota.

.LONG TERM...(Tuesday through Sunday)
Issued at 310 AM CDT Mon May 21 2018

There are several chances for thunderstorms starting late Tuesday
and lasting through Friday. At this point it appears that the main
threat for Tuesday/Wednesday will be heavy rain, while Thursday
/Friday could see more of a severe weather threat. Everything is
very conditional at this point, so expect the forecast to continue
to change over the next few days. Don`t want the details
discussion below to give a false sense of certainty.

Tuesday night through Wednesday night - Over the next couple of days
an upper level trough will dig over the Great Basin, and this does
appear to be tied to the polar jet. However, the polar jet will
ridge out across Canada, so this upper level wave will slowly fill
and rely on the weak, subtropical jet to advect its positive
vorticity downstream. On Tuesday night a shortwave trough lifts
northeast and induces low level theta_e advection over the Upper
Midwest.

Increased pops for Tuesday night across southwest Minnesota.
Forecast models are in good agreement with precipitation developing
on the leading edge of this warm air advection. Pwat values will be
around 1.5 inches, which is near the 90th percentile for late May.
Meanwhile winds within the entire troposphere are less than 50 kts,
and winds are less than 20 kts from the surface up to 400 mb. CAPE
profiles are thin, since at this time there is not an elevated mixed
layer to work with. The end result of this set up should be
overnight convection Tuesday night lingering into Wednesday morning,
and the main threat from these storms is heavy rain. We have been
dry, so at this time the antecedent conditions do not favor flash
flooding, but folks along the Minnesota River Valley and I-90 could
have a quick 2 to 3 inches or more with any one particular storm.
This convection should weaken and eventually dissipate on Wednesday
as the forcing from the low level jet dissipates.

Wednesday and Wednesday night are of lower concern. There will
probably be some lingering convection across the region, but there
shouldn`t be enough CAPE/Shear for it to be severe. It`s difficult
to pin down a persistent area of forcing, so will just continue with
the blended forecast.

Thursday through Friday - The atmosphere changes across the region
on Thursday as an elevated mixed layer seen via the H700-500 lapse
rates lifts up from the southwest. Meanwhile surface temps and
dewpoints increase beneath this EML, so afternoon CAPE values of
3000 to 4000 J/kg develop. Based on the forecast soundings, this
instability should remain capped off on Thursday so although there
are pops in the forecast, they likely will not be realized until the
evening when the first of two shortwaves drops down from Canada. If
this shortwave trends quicker like the ECMWF, then the convection
could develop sooner.

Deep layer wind shear is still on the low side, but sufficient for
organized convection. The primary threats will be large hail early
on in western Minnesota, and then transitioning to a damaging wind
threat Thursday night as the storms move eastward toward Wisconsin.
CIPS severe analogs have been showing a signal for severe storms
during this time period. Per coordination with SPC, there are still
uncertainties in timing, so at this point confidence is not high
enough to highlight a specific area in the day 3/4 outlook.

Friday is also uncertain. There is potential for storms redeveloping
as the second shortwave trough and cold front sweep through the
region. However it is certainly possible that Thursday night`s
storms scour out the region of any instability and accelerates the
passage of the cold front.

Looking ahead, it does appear that drier air will move across the
region for the Memorial Day weekend as high pressure builds off to
the east. Could see some lingering showers on Saturday, but Sunday
and Monday look dry.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1205 AM CDT Mon May 21 2018

MVFR cigs were along the Iowa border early this morning with
areas of IFR/LIFR cigs in central/western Iowa. As a disturbance
moves slowly across Iowa overnight, lower cigs will begin to
spread northward and lower into the MVFR range before 12z for
most of the southern one quarter of Minnesota. Eventually patchy
IFR cigs along with patchy MVFR vsbys in light showers will occur
in western, southwest/south central Minnesota. Some of the showers
will move across the rest of southern/central Minnesota, as well
as west central Wisconsin by the late morning/afternoon, but the
activity should decrease in overall coverage. The MVFR/IFR cigs
will likely hold across southern Minnesota during the
afternoon/evening, with the possibility of IFR/LIFR cigs after 6z
Tuesday. Winds will be generally from the east/southeast through
18z, and become light and variable overnight.

KMSP...

VFR conditions are likely through 15z, with some patches of MVFR
cigs developing after 15z. However, the normal diurnal rise in the
cigs should develop in the afternoon, with a low chance of MVFR
cigs during this TAF period. Isolated showers will be possible
from late morning, through the afternoon. However, amounts will be
less than a few hundredths of an inch. Winds will be generally
from the east/southeast through 18z, then become light and
variable overnight.

/OUTLOOK FOR KMSP/

TUE...MVFR cigs early, then VFR. Wind SE 5 kts.
WED...VFR with chc SHRA/TSRA late. Wind SE 5-10 kts.
THU...VFR. Chc -TSRA. Wind S 5-10 kts.


&&

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

$$

SHORT TERM...JLT
LONG TERM...JRB
AVIATION...JLT



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