Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
618 am CDT Monday Aug 31 2015
Update...for 12z aviation discussion below
Short term...(today and tonight)
issued at 403 am CDT Monday Aug 31 2015
Seeing stratus and light fog development again overnight...
especially from central and south/east central Minnesota into northern WI.
The deck is not quite as robust as previous nights...likely given
the better boundary layer mixing resulting from the stronger
pressure gradient with the surface trough just to the west.
However...still expect to see the low clouds linger through around
15z...with sun breaking way for the afternoon hours. Still might be
a thin layer of smoke aloft detectable for the eastern half of Minnesota
and WI...but no impact at the surface is foreseen.
The cold front associated with the aforementioned trough will sag
into west/north central Minnesota this afternoon. Brisk southerly surface
winds will develop ahead of this feature...with 15g25 knots speeds
common during the afternoon. Temperatures should moderate a few
degrees from Sunday highs...given similar 925-850 mb temperatures...but
more efficient mixing with the stronger pre-frontal southerly winds.
There really is not much in the way of forcing along the front this
afternoon...and given the 700 mb-850 mb warm nose...do not expect shower
and thunderstorm development as far south as the mpx County Warning Area. After dark
however...do see height falls into central Minnesota as shortwave energy
rides across northern Minnesota. Could envision a few showers/ts sneaking
as far south as Lake Mille lacs to Ladysmith late tonight. Have
included 20 probability of precipitation for those areas.
Long term...(tuesday through sunday)
issued at 403 am CDT Monday Aug 31 2015
The extended period continues to look warm and mostly dry through
the week...with showers and thunderstorms along a passing cold front
for the weekend. The warmth is rather impressive for this time of
year...especially when looking at the low temperatures at msp. Only
twice has there been a 4-day streak in September of low temperatures 70deg
or warmer...and currently that is forecast to occur the 2nd through
the 5th. The previous September warm spells were in 1931 (6 days)
and 1960 (5 days).
The upper level Patten characterized by a broad trough along the
West Coast and ridge over the Mississippi River basin will amplify
over the next several days. This will reinforce the thermal ridge
across the Dakotas...and bring seasonably warm air across the upper
Midwest. The prolonged low level southerly fetch will also increase
the boundary layer moisture...with dewpoints near 70 expected by middle
week. This will limit the nocturnal cooling as mentioned above...so
overnight lows will also be in the upper 60s to near 70.
Did not make significant changes to the precipitation chances because there
is still a signal for a few isolated thunderstorms each day...plus
did not want to waffle back and forth from a dry to wet forecast.
However...most locations will not see measurable rainfall through
the end of the work week.
Looking ahead...showers and thunderstorms will develop this weekend
as a cold front moves through the region...but the exact
location/timing is still in question so still only have a 40 percent
chance of rain. This should increase throughout the week as models
come into better agreement. The reason for the variability stems
from a weak shortwave with origins tied to a tropical disturbance
that is currently expected to become a tropical cyclone over the
next 48 hours as it lifts northward into the Baja California. This wave will
eventually move northeast across the upper Midwest this weekend.
However...given the uncertainty with strength and movement of this
tropical system...not to mention how it interacts with the West
Coast trough...cant latch on to one particular solution. At this
time the best chance for storms on Saturday will be west of I-
35...spreading eastward Saturday night into Sunday. The main hazard
would be potential for heavy rain as the precipitable waters climb near 2
inches...and mean layer wind (storm motion) becomes parallel to the
advancing cold front.
Aviation...(for the 12z tafs through 12z Tuesday morning)
issued at 618 am CDT Monday Aug 31 2015
Another morning of IFR stratus and fog for most sites...with
anticipated scattering similar to the other mornings /14-15z/.
After that...expect VFR by the noon hour with only some high-level
smoke potentially generating a scattered-broken 20kft-25kft ceiling. Southerly
winds become gusty today /15g25kt/ as the surface pressure gradient
tightens with the approaching Dakotas trough. Winds diminish this
evening with the potential for br again Monday night...primarily for
WI sites /krnh and Keau/ east of the broken deck of middle/high clouds.
IFR ceilings expected until 14/15z...then VFR by late morning...when
southerly winds begin gusting 22-25kts. Speeds diminish tonight
although fog redevelopment is not anticipated at this point given
increase of middle/high clouds from west.
/Outlook for kmsp/
Tuesday night...VFR. Winds south-southeast 5 kts.
Wednesday...VFR. Slight chance -tsra. Wind S 10-15 kts.
Thursday...VFR. Wind S 10-15 kts.
Friday...VFR. Slight chance -tsra. Wind S 10-15 kts.