Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
640 am CDT sun Jul 5 2015
Short term...(today and tonight)
issued at 346 am CDT sun Jul 5 2015
Concerns in the short term are centered around convective
trends today and tonight along with the threat of severe weather
late today and early tonight. Overall confidence in the forecast
remains pretty high but there have been some changes to the
forecast with regards to thunderstorm chances for today.
Early this morning...scattered thunderstorms were across eastern South Dakota.
They are located in the axis of low level moisture transport along
with elevated instability. They are slow to move east with the 850-
300mb thickness pattern almost north-S along the South Dakota/Minnesota border.
However...as the morning progresses the low level moisture
transport will work into western Minnesota and eventually into eastern
areas this afternoon. Although there will be a weakening in the
low level jet this morning as mixing ensues...various cams suggest
some scattered to widely scattered activity will progress into our
far western County Warning Area. This activity should diminish or end by late
morning...tied to the weakening of the low level jet.
Therefore...some chances probability of precipitation are now in place across the west
The convective trends for this afternoon have lower confidence as
several things come into play. First...the aforementioned morning
activity may end up festering across central Minnesota and then reintensify
this afternoon from the Twin Cities on northward. This is the
scenario depicted in the arw and nmm as well as the experimental
hrrr. The latter solution GOES as far as developing things all the
way back to Redwood Falls. This is plausible as the low level
moisture transport will lie across central and southern Minnesota this
afternoon along with building cape. Possibly offsetting this will be
a building cap. In addition...the westerly flow in the 700 mb-500 mb layer
contains several perturbation that will be moving across the area
during the day aiding in the forcing. So...a change made overnight
was to add chance to likely probability of precipitation from the metropolitan on north in the late
afternoon hours today with chance probability of precipitation back to the southwest.
Complicating matters today...which could derail all of the
above...are thunderstorms over eastern Montana. Earlier runs in the night
of the hrrr showed a strong bowing complex of storms moving into
west central Minnesota during the early afternoon hours. The latest run has
weakened this trend...but this will need to be watched.
Otherwise...by late this afternoon a cold front should meander from
north central Minnesota through eastern South Dakota and western
Nebraska. Storms should intensify along the northern portions of the
front...from Aberdeen South Dakota through International Falls. Cams are
suggestive that a bowing complex of storms may work southward across
our far western County Warning Area early in the evening with damaging winds. A
second area of strong to severe storms will likely develop a little
later southwest of Aberdeen. These storms may race eastward during
the late evening hours ahead of a deepening low pressure system and
cause another bout of severe weather across southern Minnesota. Damaging
winds were mentioned in the forecast across the west and south with
confidence being pretty high.
In addition to the severe weather threat...there remains a concern
for heavy rainfall tonight. Precipitable water values tonight are
forecast to be around 2.25 inches from west central into northeast
Minnesota. Sref probabilities for greater than 2 inches of rain is
in this same area and has remained quite consistent for several days
now. Our quantitative precipitation forecast grids for tonight indicate a large area of over 2
inches of rain across west central Minnesota with 1.25 to 1.75 across
central Minnesota. Amounts dwindle to between a quarter and a half inch
overnight from Albert Lea to Eau Claire. However...these areas will
see a good amount of rain on Monday as the front moves through.
Finally...because of the varying convective trends today...the high
temperature forecast of middle to upper 80s may be on the low side
based on 850mb temperatures and the fact that the fire smoke has
really diminished aloft. West central Minnesota is the most likely area for
higher highs with lower to middle 90s possible if more sunshine
occurs associated with less thunderstorm approaching/developing.
Long term...(monday through saturday)
issued at 346 am CDT sun Jul 5 2015
Main concerns in the long term deal with timing the departure of
precipitation chances on Monday...the strength/placement of a shortwave Thursday
night/Friday...followed by how quickly the subtropical ridge builds
in over the SW Continental U.S. Over the weekend.
12z Monday we will have a cold front bisecting Minnesota from about Sioux
Falls to Duluth...with most guidance indicating around a 1002 mb low
over central Minnesota along this boundary. We will be starting out with
the thunderstorms from tonight moving into western WI...likely in a
decaying phase. Depending on how well defined the surface low is...we
could have a good deal of stratiform precipitation trailing the front in
western Minnesota in the deformation region west of the low. For the rest of
the day...we are seeing some timing differences with how quickly
precipitation moves out...with the European model (ecmwf) hanging precipitation back a bit slower
than the NAM/GFS. Favored probability of precipitation on Monday toward the faster
solutions. Current thinking is that we will see rather extensive
showers/thunderstorms push across the region through the
morning...which will in turn push afternoon activity mainly east of
an Albert Lea to Ladysmith line. Not expecting much Post frontal
precipitation either as the surface low will be pushing across Lake Superior
during the afternoon...so that wrap around shower activity will be
northeast of the area as well by the afternoon.
In the wake of this front...models continue to show a 1020mb high
working down out of Canada...with dewpoints back down in the 40s
across the area by Tuesday morning. In fact...current forecast for
lows Tuesday morning is a bit higher than what the raw model blend
has...which is upper 40s across central Minnesota.
Otherwise...Tuesday will be a great day to skip work and do
something outside as winds will be light...skies sunny with highs in
the lower 70s...which is about 10 to 15 degrees below normal. About
the only potential blemish for Tuesday is the threat for smoke to
accompany the Canadian high once again.
For the rest of the week...we will see temperatures gradually warm
back closer to normal by Friday. There a few chances for precipitation
mentioned Wednesday night and again Thursday night/Fri...though support for
said probability of precipitation continues to look weak...especially for anything midweek.
The Thursday night/Friday precipitation chances are driven by the GFS/Canadian...
which show potent little shortwave coming out of The Rockies and
working toward WI. The European model (ecmwf) has this wave as well...but drives it
south across middle Mississippi Valley.
The weekend still looks to bring warmer...more humid...and
potentially more active weather as a sub tropical ridge starts
building to our SW. As it does so...energy from the closed upper low
currently west of California will be working across the region this
weekend. Lots of differences in the details...but both the GFS/European model (ecmwf)
would support highs working back up into the middle/upper 80s with
dewps around 70 for next weekend. Question for precipitation will become
timing/placement of waves running around the ridge...but there will
certainly be increasing potential for some healthy bouts of
primarily nocturnal convection.
Aviation...(for the 12z tafs through 12z Monday morning)
issued at 640 am CDT sun Jul 5 2015
Scattered rain showers and isolated thunderstorms and rain will work across western Minnesota
through middle morning before dissipating. Vcsh used at krwf and
kaxn. Redevelopment is expected to the west and north of the Twin
Cities during the late afternoon. Some of this convection may move
across the Twin Cities and into western WI before weakening in the
early evening. Gusty south winds up to 25 knots at the taf sites
this afternoon. The main show still appears to be strong to
severe convection developing late today along a cold front from
north central Minnesota through central South Dakota. This activity should impact
kaxn...kstc and krwf in the early evening. Strong winds are
expected with the convection with gusts of 40-45 kts inserted into
the tempo groups as the storms Bow southward across western Minnesota.
Eventually this activity will reach into eastern Minnesota and western WI
during the overnight hours. By this time...more of a heavy rain
event is expected to be underway. Due to the very moist conditions
developing tonight...the aforementioned taf sites will likely have
low MVFR or even IFR ceilings heading into Monday morning.
Kmsp...scattered rain showers/isolated thunderstorms and rain is possible during the late
afternoon and evening...especially to the north and east of the
metropolitan. Widespread rain showers/thunderstorms and rain during the overnight hours and early
Monday morning. Ceilings/visibilities during these periods will be MVFR
or lower. Gusty south winds up to 25 knots this afternoon.
/Outlook for kmsp/
Monday afternoon...VFR with rain showers ending. Winds northwest 15-20 kts.
Tuesday...VFR. Winds north-northwest 5-10 kts.
Wednesday...VFR. Winds S 5-10 kts.