697
FXUS63 KMPX 140550
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1250 AM CDT Wed Aug 14 2024

.KEY MESSAGES...

- Our dry period ends tomorrow with widespread showers expected
  to gradually spread west to east tomorrow. The heaviest rain
  is expected tomorrow evening through early Thursday morning.

- Lingering showers are possible Friday & Saturday before we
  resume our dry and mild stretch by Sunday.

&&

.DISCUSSION...
Issued at 252 PM CDT Tue Aug 13 2024

A broad cumulus field has blossomed over the northern plains this
afternoon with cumulus evident from southern Canada all the way
south through Illinois, Indiana, and Ohio including most of
Minnesota & Wisconsin. There are some high clouds moving through
southwestern Minnesota as well evident on GOES satellite imagery,
with a small chance for a few pop showers much like yesterday in
southwestern Minnesota. Light southerly to southeasterly winds
continue ahead of the incoming system which will bring showers back
to the region tomorrow, alongside a northwesterly wind shift that
looks to persist through the next several days. The impetus for
initial showers arriving in western Minnesota as early as sunrise
tomorrow is a strengthening low level jet bringing an influx of
moisture and warmth to force showers as the leading edge of a trough
begins to slide eastwards over the northern CONUS Rockies. This jet
will continue to pump gulf stream moisture northwards allowing for
increasing PWATS into the 1.7-1.9 range which is already a couple
tenths of an inch above the 90th percentile for August 14th. A
series of shortwave impulses are expected to spread eastwards from
the approaching trough which should couple with the jet forcing to
produce a large swath of showers and potential storms which will
gradually move west to east across Minnesota and into Wisconsin
tomorrow. Model guidance has a bit of spread in regards to timing
and amount of precipitation with the CAMs favoring a slower onset
time and less overall precipitation compared to the medium to long
range global models, with significant spread in the GEFS in regards
to both total amount of rainfall and the timing. The GEFS mean for
MSP would produce an inch and a half of rainfall through 7pm
Thursday with a lack of consensus within guidance as most membership
spreads from a few tenths to as much as 2 inches. The higher amounts
would be possible during the heaviest rainfall which looks to be
late tomorrow through around 5-6am Thursday as the low level jet
once again strengthens and amplifies the forcing from the trough.
The surface response is fairly muted with a surface low only forming
as the trough axis swings through by midday Thursday. Overall,
expect most of the area to pick up anywhere from 3/4 of an inch to
around an inch and a half of rain over a 36 hour period from when
precipitation enters the region tomorrow through Thursday.

Taking a deeper dive into severe weather parameters, there is also
quite a bit of spread in potential as the higher pwats and fairly
saturated moisture profile would support heavy rain but only weak
storms, especially overnight. As CAM guidance has trended slower
than global guidance, the best chance for stronger storms would be
near sunrise Thursday with a strong low level jet present alongside
increasing instability. As of right now, the main threat would be
heavy rain producing outflow or micro-burst type winds, with that
threat looking on the lower end of things for now.

As the upper level system rotates over the Great Lakes heading into
the weekend, further light showers are possible as both aloft and
surface winds shift northwesterly, with cold air advection showers
possible until the system rotates out further by Sunday. Once we get
through Saturday, the rest of the forecast looks dry and a return to
what we have been seeing the last week or so, which is mild
temperatures and dry weather.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1242 AM CDT Wed Aug 14 2024

Initialized with VFR and light winds tonight, but patchy fog has
already developed in western WI (specifically EAU). Fog may be
possible at other sites outside of MSP but confidence not high
enough to include at other sites at this point. Will continue to
monitor through the early morning hours for possible TAF
updates. The main focus of these TAFs remains the rain for
Wednesday evening through Thursday. Rain will develop from
southwest MN and progress eastward. As rain starts, a drop to
MVFR is expected and eventually some IFR is expected as well.
The coverage of IFR is still uncertain, but confidence has
increased such that IFR ceilings were added for western and
central MN sites. Rain chances will continue past the end of
the current TAFs with degraded conditions likely continuing into
Thursday morning.

KMSP...Initialized with VFR conditions and light winds which
will persist through early Wednesday afternoon. Conditions then
deteriorate late Wednesday afternoon onward as the rain shield
moves into eastern MN. Still a little bit of wiggle room as to
the start time so have maintained the PROB30 mention for a
potential of rainfall to move in prior to 00z but confidence
increases significantly from 00z onward that rain will move
across the MSP airspace. Low confidence on CB/TS so have omitted
such mention at this point. Regardless, MVFR conditions at best
will persist through Wednesday night, possibly dropping to IFR
by Thursday morning. Will see how the model guidance develops
such a scenario for later TAFs.

/OUTLOOK FOR KMSP/
FRI...VFR, chc MVFR/-SHRA. Wind NW 10-15 kts.
SAT...VFR. Wind NW 10 kts.
SUN...VFR. Wind NW 5-10 kts.

&&

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

&&

$$

DISCUSSION...TDH
AVIATION...JPC