Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1105 PM CDT Sat Mar 28 2015
Short term...(this evening through sunday)
issued at 345 PM CDT Sat Mar 28 2015
Two main weather concerns over the next 24-30 hours. First...as
precipitation begins to develop across Minnesota this evening...will the
precipitation type change over to a mixture of sleet/freezing
rain/snow or rain in wc WI. Secondly...how strong will the winds
gust as boundary layer winds increase to over 50 kts later tonight
in SW/wc Minnesota...and shift to the northwest Sunday morning...but remain
nearly 50 kts above this boundary layer in wc/SW/SC Minnesota.
Forecast soundings across wc WI remained very close to 0c nearly
5-8k above the surface as the atmosphere saturates later tonight. Only
far eastern counties in mpx County Warning Area does the sounding get cold enough
for snow. Later shifts will need to monitor the atmospheric column
in the lowest 5k and see if once the precipitation starts...does
the sounding saturate to have sleet/snow or freezing rain. Too
many uncertainties to limit all snow or sleet/freezing rain/rain.
I Don/T expect any icing problems due to very mixed atmosphere in
the boundary layer due to the strong winds. However...some of the
low spots which are sheltered from the strong winds may drop below
freezing. Too many scenarios...so will keep things simple with a
chance of all four types. In addition...quantitative precipitation forecast amts are not high
enough to warrant icing problems...plus temperatures should rise above
freezing by 8- 9 am.
The precipitation field overnight looks similar to previous
forecast with the bulk of the precipitation between 6-15z...and
mainly across central Minnesota due to the orientation of the low level jet and
moisture axis/advection. Still...quantitative precipitation forecast amts will remain low due to
the fast nature of this system...and limited precipitable water values.
As for wind...strongest winds above the boundary layer will occur
late this evening along the Buffalo Ridge. This area will likely
see gusts over 40 miles per hour by midnight. However...the pressure gradient
lessens after midnight...or before the main cold front moves into
the region. After 6 am...winds will once again increase and shift to
the W-NW. The strongest boundary layer winds occur before
noon...with a slow decrease during the afternoon hours. The main
dilemma is whether enough mixing in the boundary layer in the
morning to generate 50 miles per hour winds...especially in the typical areas
of wc/SW/SC Minnesota. Have issued a Wind Advisory based on sustained
winds of 25-35 miles per hour in these areas. There remains a chance that
winds will gust over 50 miles per hour during the morning. As the main cold
front moves across the area...precipitation chance/S will decrease
rapidly from west to east. Only central Minnesota and portions of ec Minnesota
may see some wrap around moisture to generate scattered showers
through the early afternoon.
Long term...(sunday night through saturday)
issued at 300 PM CDT Sat Mar 28 2015
Behind a potent cold front passing through the region sun...a drier
and cooler airmass with strong northwest winds will move into the region
Sun night into Monday morning under solid northwest flow. This high pressure
respite will be brief as another small low pressure center looks to
drift through the region late day Monday into Monday evening...possibly
bringing a few -shra to northern portions of the mpx County warning forecast area. There may
very briefly be some -shsn mixed in Monday morning and again Monday evening...
but nothing of significance. Behind this system...a large upper level
ridge building over the western Continental U.S. Will shift eastward...encompassing
much of the central Continental U.S. Tuesday into Wednesday. Underneath it will be surface
high pressure for Tuesday...allowing for a noticeable warming trend for
the first half of the workweek. Highs Monday-Tuesday will range from the
middle 50s east to the middle 60s west. Going into Wednesday...the County warning forecast area will
find itself within the warm sector of an approaching low pressure
system as a warm front lifts to the north and a cold front trudges through the
Dakotas. This will allow highs on Wednesday to climb to the 60-70 degree
range. While this warming trend will certainly be welcome...the
potential will be there for isolated to scattered thunderstorms over southern portions
of the County warning forecast area Wednesday afternoon. The strong southerly surge will not only bring in
the warmer air but also allow Gulf moisture to be brought up north
to the tune of dewpoints in the middle-upper 40s...raising MUCAPE
values to around 1000 j/kg with a breaking down of capping aloft.
This may allow for a few rumbles of thunder within more organized
clusters of showers. Not looking for a severe weather threat but
we are getting into Spring so thunderstorms will become more
commonplace. As this system pushes through...the large-scale setup of
trough east and ridge west will once again develop for the latter
portion of the week. In conjunction with this will be a pair of
cold fronts dropping through the central Continental U.S. Within pronounced
northwest flow aloft between the large upper-level features. The first
front will push the instability out of the region Wednesday night while
the second starts up noticeable cold air advection Thursday morning which will last through
the rest of the week. Even with the passage of the cold fronts...
several upper level disturbances will ride southeastward within the upper
level flow...each of which look to be capable of at least
producing isolated showers across the area for Friday and Friday night.
Nothing of consequence...but enough where at least slight chance probability of precipitation
are warranted. Highs for the end of the week will fall back into
the 40s with lows in the 20s and 30s...so there is some small
potential for a rain/snow mix in the overnight periods.
Aviation...(for the 06z tafs through 06z Sunday night)
issued at 1105 PM CDT Sat Mar 28 2015
Minor timing changes with regards to the precipitation with
the incoming cold front. Still appears to come in two
stages...08z-12z and then from 14z-19z. Confidence is increasing
on MVFR ceilings Sunday morning and come broken-ovc012-015 was
inserted. Three concerns remain overnight. The first is the
potential for low level wind shear...especially in the area from
krwf to kstc on east through the Twin Cities. A core of 50 knots
winds near 2k feet is seen moving northeast across the
aforementioned areas in the 09z-12z time frame. Little directional
change is noted and with surface winds already forecast near 20
kts with gusts into the 20s...kept the mention of low level wind shear out of the
taf. The second concern is the threat for snow...sleet and light
freezing rain for the WI taf sites Sunday morning. With confidence
on ptype not high...kept it to a mix of just rain and snow. The final
concern is the strong northwest winds developing behind the cold front
by late Sunday morning. Confidence remains high for near 30 knots
winds from kaxn through krwf along with gusts approaching 40
knots. Confidence is increasing late this evening that the
stronger winds may impact eastern areas of Minnesota and western WI.
Hence...wind speeds were raised a little at kmsp and krnh.
Kmsp...still expect a band of light rain to move through before
daybreak with the threat of showers lingering through the morning.
Confidence is increasing on MVFR ceilings in the middle to late
morning hours. Also bumped up wind speeds a few knots...mainly in
the gusts for Sunday afternoon with near 40 knot gusts possible.
/Outlook for kmsp/
Monday...VFR. Winds west 5-10 kts becoming SW.
Tuesday...VFR. Winds northwest 10 kts.
Wednesday...VFR. MVFR/-shra possible in the aftn/eve. Winds S 15-25 kts.
Minnesota...Wind Advisory from 9 am to 6 PM CDT Sunday for mnz041-047-048-
Wind Advisory from 11 am to 6 PM CDT Sunday for mnz042-049-057-
Fire Weather Watch from Sunday afternoon through Sunday evening