Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
844 PM CDT Thursday Aug 27 2015
Short term...(this evening through friday)
issued at 405 PM CDT Thursday Aug 27 2015
Convection continues to slowly meander down the Minnesota river
at middle afternoon...associated primarily with an mesoscale convective vortex that pushed in
from South Dakota late this morning. Some of the rainfall has
been heavy with 1.61 inches this afternoon at Tracy. The activity
will gradually diminish while heading eastward for the remainder
of the afternoon as it moves away from the better instability and
low level moisture transport. Some showers and isolated
thunderstorms will likely continue across west central areas into
the evening as middle level frontogenesis holds across the area along
with increasing low level moisture transport.
The forecast for tonight and Friday is a messy one. The
deterministic models and cams continue to oscillate north and
south with the incoming low pressure system and the resulting
axis of heavy quantitative precipitation forecast...gyrating between southern Minnesota and Iowa. One
complicating matter is that this is a phasing system in the upper
levels. One part is over South Dakota this afternoon with the
other over Nebraska. This is evident in the regional radar. These
two features will come together late tonight across southern
Minnesota and northern Iowa and then move slowly off to the east
Friday and Friday night. Our belief is that the heaviest rainfall
from this system will be along and south of the I-90 corridor.
This is at the nose of the low level moisture transport and strong
differential divergence. Rain amounts along I-90 may reach 2
inches but it will be spread out over a 6 to 9 hour period.
Hence...the threat of flash flood is low with ponding of water in
low areas more common. Still...the rainfall with this system is
definitely a concern with the upper levels originating from
Baja California/Mexico several days back. The subtropical jet wraps
anticyclonically across the upper Mississippi Valley with a rather
pronounced right entrance region late tonight and early Friday. In
addition...the precipitable water values are prognosticated to reach 1.75
inches across the southern forecast area...which is near the maximum moving
average of 1.83 inches from the kmpx sounding climatology for late
August. Rain amounts from this system will diminish significantly
heading northward...with about a third of an inch forecast for the
Twin Cities with no precipitation at the far northern end of the
forecast area (staples).
As for the lows and highs in the short term...little change for
tonight with around 60 to the lower 60s. Highs on Friday remain
quite cool across the south and east sections of the forecast area due to the
clouds and rain with middle 60s common. Highs will be near 75
back towards kaxn and kmox.
Long term...(friday night through thursday)
issued at 405 PM CDT Thursday Aug 27 2015
The main feature during the extended is still the arrival of an
upper ridge over the weekend that will persist across much of the
eastern Continental U.S. Through next week. After rain moves out Friday
evening...it looks to be hard to come by until Labor Day weekend.
Beside that...highs in the upper 80s/lower 90s are looking more
and more likely by the middle of next week.
The wave heading for southern Minnesota will be on the way out Friday evening.
The 27.12 European model (ecmwf) has come in faster with the movement of the precipitation
with this system...coming more in line on timing with the GFS/NAM.
As a result...sped up the departure of probability of precipitation Friday evening...with
dry conditions now forecast after 6z Saturday across the entire
Behind this wave...not much has changed with what the GFS/European model (ecmwf)
have been showing for the past few days with an upper ridge
building up across the upper MS River Valley as deep troughing
builds down the Pacific northwest coast. By Monday 500 mb heights across the mpx
area will build to over 588 dm. The European model (ecmwf) eventually shows these
heights nudging back down some by Friday...but the GFS keeps the
high heights centered over the area through Sunday.
This ridge will make it difficult for fronts to get in here. Not
until the end of the week does the European model (ecmwf) show a front working into
the area...coincident with the 500 mb heights starting to fall some.
Do have a couple of small thunderstorms and rain chances during the overnight
periods Wednesday/Thursday nights. The European model (ecmwf) has a couple of weak waves
working around the periphery of the upper ridge with a weak low level jet
juxtaposed over the area as well...but we will be dealing with
capping issues due to warm middle level temperatures and weak surface based
forcing. Do not think we really have much of a chance for seeing
precipitation here until the ridge starts to break down...which is Friday
on the European model (ecmwf) and not until around Labor Day for the GFS...so in
all likelihood...we are looking at the Holiday weekend before
appreciable rainfall chances can return.
On the temperature front...it certainly looks warm...though the
warmest 850 mb temperatures remain just to our west through the course of
the week. By Monday...both the European model (ecmwf) and GFS show 850 mb temperatures
warming into the middle 20s c...and with several days of sunny skies
and srly winds...we will see temperatures slowly build and by the
middle of next week upper 80s/lower 90s should rule the landscape.
Only change made to the blended forecast was to nudge highs a few
degrees higher...getting them from the low/middle 80s into the
middle/upper 80s. Beside the warm highs...dewpoint next week look to
remain in the upper 60s...so overnight lows will be remaining up
in the middle/upper 60s as well...so Don/T turn that ac off just yet!
Aviation...(for the 00z tafs through 00z Friday evening)
issued at 841 PM CDT Thursday Aug 27 2015
Tricky forecast due to a surface low pressure area expected to shift eastward across
Nebraska-Iowa overnight along with enhancing upper level features that
will expand a rain shield over much of southern Minnesota into southwestern WI
overnight through the first half of tmrw. Rain will mainly affect
krwf-kmsp-krnh-Keau...but timing and intensity will determine
flight category. Am looking for ceilings to drop into MVFR for
the southern sites overnight into tmrw morning...then conds may well drop
into IFR for a 4-6 hour period once the heavier/steadier rain tapers
off as the low pressure center drifts away. Conds improve by late day
tmrw. For kaxn-kstc...these sites look to remain too far displaced
to the north from the rain so aside from some MVFR ceilings...these
sites look to be mainly in the clear.
Kmsp...the heaviest of the precipitation looks to remain S of msp but
there will be enough moisture and upper level support to promote
prolonged MVFR ceilings and a brief window of IFR ceilings midday
tmrw. Not entirely confident on this scenario given the surface low
track...but MOS guidance is strongly identifying with this idea.
Any IFR ceilings look to be after the morning push but before the
afternoon/evening push so based on this timing the effects should not be
/Outlook for kmsp/
Sat...VFR. Winds southeast 5-10 kts.
Sun...VFR. Winds S 5-10 kts.
Monday...VFR. Winds S 5-15 kts.