Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
700 PM CDT Tuesday Sep 2 2014
Short term...(this evening through Wednesday night)
issued at 226 PM CDT Tuesday Sep 2 2014
Fairly benign first 12 hours of this period will then be followed
by a potentially active next 24 hours.
High pressure centered over the middle miss River Valley with its northern
extension over the upper miss River Valley will shift off to the east
overnight. This will result in mainly high cloud coverage for the
weather forecast office mpx County warning forecast area...though some isolated showers/thunderstorms may drift eastward over
northern Minnesota and stay away from central-southern Minnesota. Winds will back from west
to SW and S...allowing another surge of warm and moist air to this
region...bringing a brief taste of middle-Summer back to the region
A potent trough will move onshore the Pacific northwest tonight...shifting across
the northern rockies slowly Wednesday into Wednesday night. This upper trough will help
strengthen and push a low pressure center eastward across the northern plains through
the Dakotas Wednesday and Wednesday night. Attendant from this low pressure center
will be a trailing cold front off to the SW. The main concern however
will be a northeastward moving warm front in advance of the low pressure center.
Strong warm air advection may aid some shower/thunderstorm development over southwestern Minnesota in the
morning but this warm front will create much warmer/moist air to the S...
including dewpoints hitting the lower 70s. Strong surface convergence
coupled with difluent flow aloft with the warm front should help break
capping and promote storm development starting in the afternoon over
central Minnesota which should gradually translate eastward through the rest of
the afternoon into Wednesday evening. Models indicate heightened deep layer shear
which may lead to supercellular development...and this is hinted
at by large looping hodographs from the GFS and NAM. Coverage of
thunderstorms looks to be much of central and south-central Minnesota...
though the best coverage of severe will be central Minnesota...either
side of the I-94 corridor...including all of the Twin Cities
metropolitan. The deep lift of any thunderstorms will make large hail a primary
hazard...particularly with low wbz heights...but the deep layer
shear will also be conducive to producing isolated tornadoes as 0-3km
helicities are in the 350-500 m2/s2 range. Convective activity
which will be started by the northward-moving warm front may be highly
dependent on how much insolation that can be received between the
activity in southwestern Minnesota and the afternoon breaking of the cap. Convection
looks to continue through much of Wednesday evening then gradually dissipate
and shift off to the east Wednesday night. As mentioned...the surge of
warmer air will push highs into the low- middle 80s...which when
combined with dewpoints around 70 will push heat index values into
the middle-upper 80s. With the cold front still to the west of the mpx County warning forecast area
through Wednesday night...this will make for a warm and muggy Wednesday night
period as lows only drop to the upper 60s to lower 70s.
One other concern is heavy rainfall...particularly for eastern Minnesota into
western WI for Wednesday afternoon and evening. For much of southeast-east Minnesota into western WI...the
past 10 days have seen those areas receive 4-9 inches of rain...
making for a very saturated ground. An additional 1-2 inches...and
possibly locally more...cannot be ruled out from this system...
especially as precipitable waters range 1.50-2.00 inches by Wednesday afternoon and evening
over eastern Minnesota and western WI. Difficult to pinpoint exactly where the
heavy rain will be due to the convective nature of the precipitation...
but having flooding rains over eastern portions of the weather forecast office mpx County warning forecast area
cannot be ruled out. It will also need to be monitored for how
heavy rain may impact main Stem rivers...particularly the Chippewa
river in western WI.
Long term...(thursday through tuesday)
issued at 226 PM CDT Tuesday Sep 2 2014
Overall the long term portion of the forecast will be
characterized by Fall-like weather...with shower and
thunderstorm chances bookending the period on Thursday and
Thursday will start off with the surface low circa 1000mb
bisecting the forecast area. Nocturnal convection may linger a
tad during the morning hours...but will likely see a break in the
activity until the passing cold front potentially Sparks
additional showers and thunderstorms during the afternoon. It
currently looks to be the most probable along and east of the
Interstate 35 corridor. However...the stronger middle/upper forcing
will be displaced to the north...and residual clouds/precipitation could
limit destabilization /especially given the prognosticated middle level
cap/...so the threat for severe storms is minimal.
From Friday through the weekend...nearly zonal flow becomes
established across the north central Continental U.S....with a broad high
pressure ridge prevailing at the surface. This will mean dry
weather with cooler than normal temperatures and light
winds...quite Fall-like weather. Highs from Friday through Sunday
will maximum out in the upper 60s to lower 70s range...while lows dip
into the 40s and lower 50s.
On Monday...a trough begins to dig over the Pacific
northwest...inducing High Plains surface cyclogenesis. Both 02.12z
European model (ecmwf) and GFS models indicate a warm front will lift into the
area late Monday and Tuesday...and facilitate fairly decent
chances for showers and thunderstorms on Monday evening and
Tuesday. Have included high chance probability of precipitation for now until timing and
future model runs can pin down when and if higher probability of precipitation would be
Aviation...(for the 00z tafs through 00z Wednesday evening)
issued at 700 PM CDT Tuesday Sep 2 2014
Elevated instability will spread into west central and southwest
Minnesota during the early morning hours on Wednesday. This will lead to
a few showers and thunderstorms being possible at krwf after 08z
and after 10z at kstc and kaxn. As the morning progresses this
activity may brush kmsp and krnh as it moves northeast. Conditions
will primarily be VFR except in the heavier showers and
thunderstorms. During Wednesday afternoon...the shower and
thunderstorm activity is expected to become more concentrated along
a line from kaxn through kstc to krnh. There are indications that
some of the storms could spread southeast into the Twin Cities and
Keau during the evening hours. South winds 5 to 10 knots tonight
increasing to 10 to 15 knots with gusts to 20 knots on Wednesday.
Kmsp...primarily VFR through the period outside of shra/tsra. The
first threat for rain showers is from 14z-18z with a threat for thunderstorms and rain
increasing tomorrow evening in the 02z-06z period.
/Outlook for kmsp/
Thursday...VFR. MVFR/thunderstorms and rain likely in the afternoon. Winds S 10-15 kts becoming west.
Friday...VFR. Winds northwest 10 kts.
Sat...VFR. Winds west 4-8 kts.