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Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
737 am CDT sun Sep 21 2014

Update...
issued at 650 am CDT sun Sep 21 2014

Updated to include 12z aviation discussion below.

&&

Short term...(today and tonight)
issued at 425 am CDT sun Sep 21 2014

Today-tonight...surface analysis shows a cold front pushing across Lake Michigan
into lower Michigan with a secondary cold front draped from central
Manitoba province through the northern Great Lakes into Quebec province.
The first cold front was responsible for the strong/severe thunderstorms that
passed through this region yesterday...while the second cold front is more
reflective of the upper level vorticity maximum dropping southeastward from S-central
Canada. Cyclonic flow around this upper vorticity within cold air aloft and
lingering deep moisture will produce scattered showers over western WI into
far eastern Minnesota through the first half of today. No severe weather will come from
this...and highly likely that no thunderstorms will develop with a much
more stable atmos in place. So...have kept probability of precipitation no higher than chance
category from daybreak on as coverage not expected to be all that
great...and quantitative precipitation forecast will be quite light...generally only up to a few
hundredths to be received. Any and all precipitation should dissipate and
shift off to the east by midday with gradual clearing expected
thereafter as high pressure slowly arrives from the west. Deep dry northwest
flow from the surface upward will promote clearing skies this evening
through the overnight hours. The only question will be the coverage of
fog overnight as cold air advection kicks into high gear while winds drop off
under the clearing skies. Crossover temperatures are close so it may well
be a matter of how much moisture can be scoured out before the
nightly inversion sets in. Have opted to include patchy fog for
northern and eastern portions of the County warning forecast area where winds are most likely to go
calm and where coldest temperatures are expected. With respect to the
cold air advection...highs today will be noticeably cooler than Saturday with highs
ranging from the lower 60s in western WI to the upper 60s to around 70 in
western Minnesota. For tonight...lows outside the Twin Cities metropolitan will drop to
the low-middle 40s...with min temperatures close to 50 within the tc metropolitan.

Long term...(monday through saturday)
issued at 425 am CDT sun Sep 21 2014

The upper Midwest will find itself on the warm side of a highly
amplified upper level pattern which will lead to temperatures that
will be well above the seasonal average for late September. Dry
weather is expected aside from scattered showers late Tuesday
through Wednesday.

Early morning water vapor imagery with GFS 21.00 500mb heights and
winds showed a digging trough over the northwest Pacific with a
ridging over the Canadian rockies. Meanwhile a weakening occluded
pv anomaly was situated across Southern California...and this
will be the focus for precipitation chances early in the period. Forecast
soundings show dry air in the lower levels...so may see more middle-
level saturation/clouds than actual precipitation. Therefore broke up the
probability of precipitation into 6hr blocks to increase the temporal resolution.
Also...forecast soundings do not show elevated instability...so
have low confidence in upright convection and as result removed
the thunder mention from the weather grids.

Looking ahead...warm air moves across the region as the upper
level pattern amplifies. Was tempted to increase probability of precipitation well above
guidance...but there are two variables that prevented this. First
of all...forecast models hint at a weak wave cutting off over the
corn belt...and this could lead to middle/upper level clouds which
will severely limit afternoon heating. Secondly...surface winds
may contain a slight easterly component. Forecast soundings show
afternoon mixing with winds eventually becoming 180-190deg...but
if clouds move in then the boundary layer mixing will be less than
currently forecast. Since it is only Sunday...have plenty of time
to raise temperatures for the latter half of the week...so was
content to stick with the higher end of guidance...rather than
exceeding it.

&&

Aviation...(for the 12z tafs through 12z Monday morning)
issued at 650 am CDT sun Sep 21 2014

Cyclonic flow around a departing upper level low showing up nicely
on WV imagery this morning is bringing down MVFR ceilings to most
taf sites this morning...plus a batch of showers gliding southward over western
WI. The -shra will be in-and-out of both krnh and Keau for the
first 2-3 hours of the 21/12z tafs along with fairly stoic MVFR ceilings
through midday while the remaining sites /except krwf which looks to
remain VFR/ will experience MVFR ceilings for at least the first few
hours of this taf set. As the upper low continues to move away to
the east and high pressure arrives from the west...skies will clear out this
afternoon into this evening...eventually resulting in sky clear. Breezy/gusty northwest
winds will persist for much of the day then drop off steadily late
day to near calm overnight.

Kmsp...ceilings bouncing between sct020 and bkn020 for the first
few hours...then VFR with gradually lifting ceilings throughout the
day. No precipitation expected at kmsp...but will not be far off to the east
through late morning. Cloud cover to gradually scatter out late
day...eventually ending up VFR this evening through overnight.

/Outlook for kmsp/
Monday...VFR. Winds west 5-10 kts.
Tuesday...VFR. Winds S 10 kts.
Wednesday...VFR with MVFR/shra/tsra possible. Winds S 10-15 kts.

&&

Mpx watches/warnings/advisories...
Minnesota...none.
WI...none.
&&

$$

Update...jpc
short term...jpc
long term...jrb
aviation...jpc

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