Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 201026

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
526 AM CDT Tue Mar 20 2018

.updated for 12z aviation discussion below...

.SHORT TERM...(Today and tonight)
Issued at 322 AM CDT Tue Mar 20 2018

The short term concern remains the timing/amounts of any
accumulating precipitation today.

Latest model trends continue to drop upper trough over the central
cwa this morning and weakening throughout the afternoon. Still see
airmass having a difficult time saturating to the east because of
the drier easterly low level flow. This should gradually give way to
saturation over most of the MN cwa through 12z. It looks doubtful
anything will work into west central WI through the entire event.
QPF amounts of 0.05 to 0.10 inches for today will give snow amounts
of an inch or less. With forcing being so weak, low rates and warm
surface/lower boundary layer temperatures, will be tough to really
accumulate much on roads.

System drops south and weakens further this evening, and any
accumulating precipitation threat should end by early evening. Mat
see some drizzle/freezing drizzle or flurries after that, but no
real forcing indicated.

Clouds will remain through tonight, maybe some erosion over west
central Wisconsin.

.LONG TERM...(Wednesday through Monday)
Issued at 322 AM CDT Tue Mar 20 2018

Wednesday and Thursday...Dry weather is expected for the middle
portion of the week as high pressure builds over the central CONUS
on the backside of what becomes another nor`easter. In addition,
drier northwest flow aloft will transition to ridging atop the
region as a large H5 ridge over the western CONUS shifts into the
central states. This will be in conjunction with a large longwave
trough moving onshore the Pacific Northwest Thursday into Thursday
night. A mix of sun and clouds can be expected as moisture streams
into the area over the incoming ridge but clouds will be mainly
confined to the upper levels. Mild temperatures are expected with
the increasing insolation and increased H7-H5 heights. Highs will
climb from the upper 30s/lower 40s on Wednesday to the low-mid 40s
on Thursday.

Friday through Monday...A highly active pattern is expected for
the end of this week through the start of next week. Complications
remain throughout this portion of the extended forecast as continued
model disagreement along with temperatures above and below
freezing make for periods of rain and snow throughout this period.
Precipitation is expected to develop Friday as the ridge aloft
flattens out over the area and several shortwave disturbances
aloft arrive in advance of the main longwave trough. Isentropic
lift will become heightened in conjunction with the arrival of
deeper moisture Friday night into Saturday, which would hint at
fairly good precipitation generation. However, the timing and
amounts of each type of p-type are highly in question as the
Canadian, GFS and ECMWF continue to show great differences in the
overall amount and placement of deepest moisture and hence QPF
(anywhere from 0.25" to 1.50" Fri- Sat) along with where the
thermal gradient will lie. If anything, there is better agreement
on having temps fall several degrees below freezing Friday night
into Saturday morning coinciding with PWATS around 0.75" for
several 6-hour periods in that timeframe. Given the absence of
warm air aloft, this would lead to periods of rain transitioning
to heavy wet snow then changing back to rain. Again, still too
much uncertainty to have a good feel as to the distribution of the
p-types, the timing of the changeovers and any snowfall amounts.
Have still maintained PoPs into the likely and categorical ranges
for late Friday into Saturday. The main area of low pressure will
remain to the south and track eastward just as so many previous
low pressure systems late this winter. Precipitation looks to wind
down Saturday night but then the main longwave trough will slide
across the Rockies, sending additional shortwave disturbances
through the central CONUS while plenty of moisture will remain in
place over the region. This will result in periodic rain/snow
showers Sunday through Monday night and even into Tuesday.
Temperatures will continue to drop below freezing at night then
close to 40 during the day early next week, thus allowing for
multiple p-types and a messy forecast.

On the whole, the impacts from these multiple days of
precipitation may range from slushy travel conditions to
localized flooding as ground is still frozen and the potential for
runoff is rather high. Due to the aforementioned model
disagreement and forecast uncertainty, all are urged to keep up
with the latest NWS forecasts and statements.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 525 AM CDT Tue Mar 20 2018

Snow is very light and not restricting visibility below 1 1/2sm
at the most intense. Expect this to gradually work east into
central MN through the morninng and should taper off during the
remainder of the afternoon. We will likely hold onto IFR cigs to
the far west - KAXN/KRWF region through the period. May even see
some fog develop near KRWF during the night. Farther east any MVFT
cigs will give way to VFR at KSTC/KMKT/KMSP during the afternoon
with VFR trend through the forecast in west central Wisconsin.
Weak E-SE winds becoming light NE or vrbl during the night.

Flurries developing as column saturates. We should see vsbys lower
to near 2SM and cigs dropping to around 1500ft at their lowest
during the morning with improving ceilings to VFR again this
afternoon and remaining so the remainder of the period. With such
low precipitation rates and relatively warm temepratures, it will
be difficult to accumulate much snow. Maybe around one half inch.

WED nite...VFR. Wind vrb 03kts.
THU...VFR. Wind SE 5 kts.
FRI...VFR early becoming MFVR with -RA/-SN. Wind E 15G25 kts.




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