Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
553 am CDT Sat Jul 4 2015
Short term...(today and tonight)
issued at 324 am CDT Sat Jul 4 2015
Boundary that slipped into central Minnesota/northwest WI last night is now all
but indiscernible...with a very baggy pressure gradient in place.
This boundary did hang up a band of lower 60 dewpoints across
central Minnesota...which is where fog has been expanding. Of course this
area already had visible restrictions Friday afternoon in smoke...which
is certainly aiding in the poor visible across central Minnesota. In fact...we
were getting reports of ash depositing in St. Cloud yesterday...so
the smoke is giving US some extra cloud condensation nuclei in the
low levels...which is further helping the fog along. May take some
time to clean up the fog given the continued dense smoke over the
area that will slow down diurnal heating.
For the rest of today...fully expect dense smoke to again lead to a
hazy sky...the question remains though as to how much surface
visibilities will be reduced. Northwest surface winds that brought smoke
into central Minnesota yesterday have disappeared...with srly winds slowly
taking hold during the morning...so do not anticipate surface based
smoke to make it any farther south than it did yesterday. Current
thinking is that fog in central Minnesota will lift into a reduced visible in
smoke that will begin to finally improve during the afternoon.
For precipitation...have removed all of it for the short term period.
Aloft...we have ridging and at the low levels we have lost any
semblance of a front. Cams have come in dry as well...so saw no
support for maintaining any probability of precipitation. This afternoon...we will see a
shortwave working across the Dakotas that will kick of scattered
thunderstorms by the afternoon to our west. Some deterministic
guidance tries to work some of this into western Minnesota late
tonight...though it is running into some pretty dry middle-level air
over Minnesota...so followed the idea of the cams in keeping any activity
tonight out of the mpx area.
For temperatures...dense smoke will again hold back highs a bit. The
last 3 days...we have seen highs about 5 degrees cooler than what our
925-850 mb temperatures would suggest and expect more of the same today.
Basically forecast a continued slow moderation in temperatures...
with highs today forecast to be 2 to 4 degrees warmer than what we
saw Friday...which was 2 to 4 degrees warmer than what was seen on
Long term...(sunday through friday)
issued at 324 am CDT Sat Jul 4 2015
Concerns in the long term continue to be from Sunday into Monday
when a vigorous short wave and surface low pressure system move
across the northern plains and upper Mississippi Valley region.
Although model trends remain consistent with these features...there
continues to be a slight slowing on the eastward progression from
run to run as deepening surface low pressure moves from central
South Dakota Sunday evening to Lake Superior by Monday evening.
The three main concerns are highs on Sunday...the severe weather
threat Sunday afternoon/night and the heavy rainfall potential
Sunday night and Monday.
Beginning with the first concern...the flow aloft should be more
westerly on Sunday and perhaps even southwesterly late in the
day. This should clear the area of the fire smoke allowing for
highs to be more representative of mix-down values. A trend noted
in our local climate products is that highs have been increasing
about 4 degrees per day since July 1st. This trends matches nicely
with forecast highs in the middle to upper 80s in Sunday...with
the warmest readings in west central Minnesota under the low level
The second concern is the severe weather risk Sunday afternoon
and night. With the continued slowing on the eastward movement of
the frontal boundary...the eastern portion of South Dakota looks
to be at greatest risk for severe storms during the late
afternoon/evening. This is the area where cwasp (craven-
wiedenfeld aggregate severe parameter) is highest along with the
sref calibrated severe probabilities. Hence...much of the day will
be dry in our forecast area with only the far western and northern County Warning Area at
risk for thunderstorms late in the day...but even these areas may
not see much until evening. So the slight risk confined to our
west central seems okay at this point with the main threat Sunday
The third concern is the heavy rainfall potential Sunday night and
Monday as the system moves slowly eastward. The NAM and GFS
continue to show precipitable water values in the 2 to 2.5 inch range spreading
eastward across the forecast area Sunday night ahead of the cold front. This
is in record territory for early July. Following the European model (ecmwf) low
level moisture transport fields as well as the 925-850mb
frontogenesis and negative fn vector divergence...points to the
western half of the Minnesota County Warning Area as having the highest chance for heavy
rain Sunday night. The aforementioned fields shift eastward on
Monday and weaken...lessening the risk for heavy rain...but not
totally eliminating it. Quantitative precipitation forecast grids for the Sunday night/Monday
periods worked out to a little over 2 inches from Canby to Little
Falls with 1.25 to 1.5 inches from the Twin Cities through west
central WI. The lowest total quantitative precipitation forecast was near the I-90 corridor with
around an inch forecast. Heavy rainfall was included in the
weather grids where the probability of precipitation were categorical.
In the wake of this system...high pressure along with cooler
and less humid conditions will spread in for Monday night through
much of the work week. An exception on the precipitation chances
is on Thursday when a small threat for showers and thunderstorms
is indicated but not supported very well by the European model (ecmwf). Highs much
of next week will be in the 70s.
Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 553 am CDT Sat Jul 4 2015
Fog developed overnight in moisture left over from where the front
that dropped down into the area washed out overnight. With the sun
starting to rise...already starting to see visibility improvements...so
do not expect 1/2sm or less type visibilities to last much beyond 13z.
Will likely have some MVFR visibilities in smoke linger through the
morning in central Minnesota...though as srly winds become
established...we should see surface visibilities improve back above 6sm by
the afternoon. Have high confidence in a dry period at all taf
Kmsp...will have occasional MVFR visible this morning in a haze/br/fu
combo...but other than that it will be VFR throughout with winds
becoming established out of the south.
/Outlook for kmsp/
sun...VFR. Thunderstorms and rain with MVFR/IFR likely late. Winds S 10-15 kts.
Monday...MVFR/IFR with thunderstorms and rain early then VFR. Winds northwest 10-15 kts.
Tuesday...VFR. Winds north 5-10 kts.