Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
642 PM CDT Monday Jul 27 2015

Short term...(this evening through tuesday)
issued at 314 PM CDT Monday Jul 27 2015

Fairly challenging forecast through Tuesday evening. A seasonably
potent shortwave will lift northwest along the international
border overnight into Tuesday. This will lead to a 990mb surface
low that will track from central North Dakota to northern Ontario.
A trailing cold front will move eastward across the region late
tonight into Tuesday. Given the strength and magnitude of this
system...together with the convection which will inevitably
develop...have difficulty trying to pin down the location and
timing of precipitation. In addition...hires models are slow to
catch on to the mesoscale convective system across Iowa...which decreases the confidence
in their solutions going forward.

There are two features that are difficult to track. The first trying
to determine where the convection will develop later tonight. Given
the storms across would think that the overnight
convection...which will be driven by the low level jet...will be
farther south than the numerical guidance have pushed
back the timing and decreased probability of precipitation over the northern half of the
County Warning Area. Meanwhile...the convection which develops along the cold front
should become oriented into a north/south line...and move eastward
during the early morning hours. The CR-NAM nest illustrates this
scenario nicely. Cape and shear should be sufficient for a few
strong storms especially as the low level jet strengthens
overnight...causing damaging wind to be the primary threat early
Tuesday morning.

In retrospect...the last few nocturnal/early morning systems have
caused the parent boundary to move through the region faster than
anticipated...and the next days convective threat has been forecast
too far west. This is important for tuesdays convection...since the
synoptic cold front will be the focus for convective initiation.
However...the difference between this system and the past few is the
notable strength of the upper level wave driving the surface
features. Given the upper level forcing for ascent...another low
level boundary (area of convergence) should develop in the wake of
the morning convection...and this combined with middle level lapse
rates of 7-8 c/km...together with increasing deep layer shear should
support severe storms Tuesday afternoon/evening. The exact location
remains uncertain...but at this time expect the best threat for
storms to be along and east of a line from alert Lea Minnesota through Rice
Lake WI.

Long term...(tuesday night through monday)
issued at 314 PM CDT Monday Jul 27 2015

The large scale pattern through the extended period looks to be
dominated by northwest upper flow as the strong shortwave moving
through over the next 36 hours closes off over Hudson Bay and
remains nearly stationary through the balance of the forecast
period. The 12z guidance is in fairly good agreement on this
scenario. Overall... this results in a fairly dry pattern for our
area through the period as the more significant
moisture/instability will be displaced south of the area.

Lingering storms that redevelop tomorrow afternoon should exit the
eastern portion of the area during the evening hours... with much
drier air working in overnight. Dry northwest flow and high
pressure should then keep things fair and dry with near normal
temperatures through Friday. A weak surface through does look to
drop through the area late Thursday/early Friday... but given
minimal moisture return ahead of it and meager middle-upper level
support... it looks like it should move through dry. Precipitation chances
return Saturday and Saturday night as a more healthy upper wave
and surface front move through the region. The GFS and European model (ecmwf)
differ a bit on timing and position of the upper wave... with the
European model (ecmwf) bringing the upper support farther south/east than the GFS.
But... the differences Don/T impact the overall probability of precipitation too much at
this point... with chances looking warranted for Saturday and
Saturday night then drying out for most of the area for Sunday. We
could see precipitation try to work back north on Monday as the baroclinic
zone lifts northward... so included some chance probability of precipitation for that
possibility. At this point... there are no signals through period
from Wednesday through Monday for any widespread significant precipitation
or severe weather across the area.


Aviation...(for the 00z tafs through 00z Tuesday evening)
issued at 642 PM CDT Monday Jul 27 2015

The first 6 hours of the forecast look pretty straight forward
with VFR conditions. Thereafter...confidence in VFR conditions
lowers due to a warm front lifting northward into central/southern
Minnesota and west central WI. This boundary should help initiate some
showers and isolated thunderstorms...but timing and location
between short term solutions vary considerably. In addition...due
to the moist low level conditions...a repeat of stratus and fog is
possible at just about any of the taf sites during the overnight
hours. This is especially true for kaxn and kstc on southward.
Overall...pushed the timing a little forward on the thunderstorms
chances late tonight and Tuesday morning. Held tight on the low
ceilings forecast for the aforementioned terminal and expanded the
MVFR into the Twin Cities as well as krnh for Tuesday morning. Southeast
winds tonight in the 10-15 knot range increasing to 15-20g25kts
for early Tuesday.

Kmsp...shra/tsra chances highest in the 11z-14z period with
lingering showers into the early afternoon. A period of MVFR
ceilings expected in the morning with very moist low level
conditions ahead of a cold front. The front will not pass the
airfield until Tuesday evening (02z). Gusty south winds Tuesday
morning (15-20g25kts) will become southwest in the afternoon.

/Outlook for kmsp/
Wednesday...VFR. Winds west at 20g30kt.
Thursday...VFR. Winds west at 10-15 knots.
Friday...VFR. Winds west at 5 knots.


Mpx watches/warnings/advisories...


Short term...jrb
long term...99

National Weather Service Glossary of Abbreviations