Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Twin Cities/Chanhassen Minnesota
646 PM CDT Sat Aug 29 2015
Short term...(this evening through sunday)
issued at 311 PM CDT Sat Aug 29 2015
As if the shorter days were not enough to show you the sun is
getting weaker as we end August...the stubbornness of the stratus
this morning should be all the proof you need that the seasons are
about to change. Add in a lack of winds and mechanical mixing and
it will take until almost 21z to finally clear the stratus out.
These light winds and weaker mixing have even impacted western Minnesota
where the sun has been out all day...but temperatures have struggled to
warm out of the 70s.
For tonight...expect stratus to expand for the third night in a row
to the west of the remnant surface trough that extends from Missouri up
toward western lower mich. Two nights ago we saw this stratus over north
central Minnesota...last night it was across Iowa up into southern Minnesota into central
WI. Tonight...the stratus will form a little farther southeast of
its previous nights location...so that will put less of the area
under the gun for seeing low stratus again. Based on sref probs for
ceilings less than 3k feet...expect threat for stratus tonight to be
greatest from along...south and east of a line from Fairmont...to
the Twin Cities and Ladysmith. Not helping the fog/stratus tonight
is the fact that we will be picking up a pressure gradient...with
more persistent south winds expected through the night...which will
help keep mixing a little better. Beside lowering the stratus threat
some...mixing from these strong winds will make any stratus we do
see tomorrow morning easier to break up than we saw today.
For Sunday...did nudge highs down a couple of degrees...mainly in
western Minnesota. 925-850 temperatures came down a nudge from what models were
showing last night. Where 90s are in the Dakotas today is
associated with 925-850 temperatures of 28c in the NAM/GFS and 26c in
the European model (ecmwf) /the NCEP models have had a warm bias all Summer/.
Tomorrow...those isotherms stay west/northwest of the mpx area...so mixed
in some bias corrected European model (ecmwf) numbers to the going forecast. This
cooled the west a couple of degrees while keeping eastern parts of the
mpx County Warning Area more or less unchanged.
Long term...(sunday night through saturday)
issued at 311 PM CDT Sat Aug 29 2015
By Sunday night...the upper ridge will be in place across our
area...extending from the southwestern U.S. Into the upper
Midwest on the 500mb height level...with 588 dm heights pushing
into southwestern Minnesota. Any semblance of a jet stream will be north
of the Canadian border through Tuesday. 850mb temperatures around 20c
both Monday and Tuesday with low level southwesterly flow spell
day time highs in the middle to upper 80s.
On Wednesday...a highly amplified trough will come onshore in the
Pacific northwest and will be the feature to keep an eye on as we
head toward Labor Day weekend. With this pattern...a mostly dry
pattern for our area through the work week...with occasional
chances of very sporadic showery and isolated thunder activity.
With the low level heat though...we will likely be capped for
several days...and even with a weak shortwave rippling through middle
week...the forcing associated with said wave may be too weak to
overcome the cap. The probability of precipitation in the current forecast include broad
slight chance probabilities...which is likely too high an
widespread given the capped environment...but at this range we
cant get too precise with the location of the best chance for a
few showers or storms to get going.
Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 646 PM CDT Sat Aug 29 2015
Not much to change yet from the previous set of tafs as there is
not enough compelling evidence either way to make significant
changes. There does remain a reasonably high probability for IFR
or lower ceilings developing over Iowa and lifting northeast across
southern/eastern Minnesota and into Wisconsin during the overnight near
the surface ridging. Short range guidance does have a good handle
on the current stratus over Iowa...and are quite bullish in
bringing very low ceilings in late tonight. Statistical guidance is
surprising optimistic of this not occurring. Will continue to
monitor this evening and amend as necessary.
Kmsp...still uncertain whether the stratus/fog will lift into
msp...but the most recent runs of hi-res models are pushing the
stratus even further west so confidence is beginning to increase for
higher impact ceilings arriving later tonight.
/Outlook for kmsp/
Mon-Wed...VFR. Winds S 5-15kt.