Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 182210

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
410 PM CST Sat Feb 18 2017

.SHORT TERM...(This evening through Sunday)
Issued at 410 PM CST Sat Feb 18 2017

The main short term concern remains temperature trends.

Surface ridge moving east across the cwa this afternoon and
overnight.  We did cool several degrees at H85 and with limited
wind/mixing we didnt warm up as quickly today. Also...some high
level clouds did manage to block the sun in several locations. We
will see light winds into the easter cwa overnight and could see
temperatures drop off. However...we will see the higher level clouds
from time to time and could limit significant a significant drop.
There will be some fog potential as well...mainly over the areas
seeing remaining snow cover...which is into portions of west central
Wisconsin and over central MN. We did mention some patchy fog
in those locations after midnight.

As the ridge retreats Sunday...increasing southerly flow is
expected...especially over the MN portion of the area.  Temepratures
should warm back into the lower and perhaps a few mid 60s to the lee
of the Buffalo Ridge. Farther east...highs will be determined by the
amount of cloud cover. We expect some mid/high level clouds once
again...which could affect maximum heating potential. Still...
readings will be near record levels once again...with the exception
of west central Wisconsin.

.LONG TERM...(Sunday night through Saturday)
Issued at 410 PM CST Sat Feb 18 2017


The system that continues to bring heavy rainfall to the southwest
will eject into the western High Plains Sunday night. The
meridional flow Sunday night and Monday will bring moisture
northward deep from the tropics on a southerly 35-50 kt LLJ. PWATs
are expected to reach nearly 1.2 inches by Monday morning, which
would be nearly 6 standard deviations above normal, and far
eclipsing the monthly record of 0.87 inches at MPX. With the
increasing moisture and dew points reaching the 50s, temperatures
will hold nearly steady Sunday night in the 50s across southern
and western MN and in the mid to upper 40s north of I-94. These
readings are likely to exceed the warm low monthly records by
several degrees at MSP, STC, and EAU.

Showers and some thunderstorms will blossom late Sunday night
across the eastern Plains as the system approaches, with that arc
of precip reaching MN early Monday morning and WI by late morning
or afternoon. Following that initial band, models are hinting at
the possibility of clearing ahead of the surface front. If a good
amount of clearing does occur, temperatures could easily rebound
into the mid or upper 60s, with 70s just to the south across Iowa.
Surface dew points in the mid 50s would contribute to MUCAPE of
500 J/kg and best LIs of -2 to -4C. This should be enough to
generate another narrow band of storms along the front. With 0-6km
bulk shear magnitudes of 50-60 kt, low topped supercells, or a
broken line of storms given the tendency for wind to become
unidirectional immediately preceding the front, would be the
storm mode(s). The main threat appears to be marginally severe
hail, although a weak tornado couldn`t be ruled out either if the
surface winds remain backed a bit longer and temperatures warm
well into the 60s. Generally would like to see a bit more
instability before talking up a decent severe threat even if it
is February, so temperatures and dew points will need to be
watched closely.

Warm air follows the early week system. In fact, thicknesses rise
Tuesday again back into the mid 550s DM with 925 mb temps of +9
to +14C. Raised temperatures again Tuesday and we are now calling
for upper 50s to mid 60s. Temperatures modify slowly midweek as
high pressure to the north builds into the northern Plains and
Upper Midwest and tightens the baroclinic zone to our south. We
could break more records Wednesday before the frontal passage
during the afternoon, but this will heavily be dependent on
precise timing of the front.

Meanwhile, a potent and compact upper wave crashing ashore
central CA Tuesday night will eject eastward to the central Plains
Thursday and begin to interact with the tightening baroclinicity.
The system looks quite impressive on satellite over the Pacific
today. Models are still bullish on developing this feature over
the central Plains and tracking it northeastward to the central
Great Lakes sometime Friday or Friday night.

Model consistency took a bit of a hit today with the GFS speeding
up significantly, the Canadian shifting south, and the ECMWF
shifting slightly south as well. Overall, still like the timing of
the Canadian and ECMWF the best and the ECMWF has been most
consistent from run to run, so heavily based this forecast on that
solution. The main culprit for the shifting appears to be an upper
low across central Canada. Yesterday this low was well off to the
west, allowing ridging to build further north into the Great Lakes
and resulting in a more northerly track across Nebraska, Iowa, and
Wisconsin. Today the upper low is further east, resulting in a
flatter pattern to the east and the low tracking along the very
impressive baroclinic zone set up roughly along or just south of
I-80. This is all in the weeds quite frankly at this range and we
can expect model shifts for a few more days. It is interesting
that there are an increasing number of GEFS members that do favor
over a foot of snow through the forecast area, now roughly a
third of them. These are also slower like the ECMWF/GEM than the
operational GFS, which is the quickest of the GEFS members.

Overall, didn`t make any drastic changes to the forecast for late
week except to temper PoPs a bit across western Minnesota. Because
of such timing differences and the shift the GEM made with the
12Z run, the standard model blend really took a hatchet to the
PoPs. Still felt confident enough to pinpoint two periods that are
very likely to experience snow (Thursday night and Friday) and
blended the previous forecast back in for continuity. The bottom
line is confidence is still quite high this system will be strong
and impact a large area nearby with heavy snow and potential
blizzard conditions. Exactly where or how much simply cannot be
resolved at this range with any meaningful accuracy.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1120 AM CST Sat Feb 18 2017

Continue the trend of VFR conditions through the period. Light
west winds will become southerly into tonight with some high
clouds move through. Expect winds to become southeasterly later
tonight into Sunday.


VFR conditions throughout. Winds should be light west becoming
southeasterly later tonight. Only high clouds expected to move
through during the period.


MON...MVFR with SHRA Likely. ISOLD TSRA. Wind S at 10 kts.
Tue...VFR. Wind SW at 5-10 kts.
Wed...VFR. Wind SW at 10G15 kts.




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