Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
606 PM CDT Monday Jun 29 2015
Update...for 00z aviation discussion below
Short term...(this evening through tuesday)
issued at 258 CDT Monday Jun 29 2015
Early afternoon water vapor imagery shows a shortwave trough diving
across Minnesota...while surface dewpoints in the low to middle 60s have
contributed to MLCAPE values nearing 1500 j/kg. Said instability
combined with decent shear values will lead toward an increase in
coverage and likely intensity of thunderstorms as diurnal heating
further destabilizes the air mass. The smoke filtering down from
Canada/eastern North Dakota across western/central Minnesota does seem as if it
will limit the growth/development potential initially...but there is
some erosion of that occurring in north central Minnesota that will likely
work into central Minnesota. Storm Mode should be multicell or even
supercellular in nature...with large hail and damaging winds the
main threats. These storms should also be efficient rainfall
producers given precipitable water values nearing 125 percent of
normal and the opportunity for cells to track/regenerate over
the same area.
Showers and thunderstorms should come to an end by midnight...as the
shortwave energy lifts out of the area and we lose the influence of
daytime heating. Clouds scatter out overnight...but winds should
stay up enough to limit anything other than patchy fog formation.
Also...expect lower /stratus/ clouds to filter into west central WI
from the northeast toward daybreak Tuesday.
Northwest middle level flow persist over the area for Tuesday...but the
forcing appears to be displaced out of the area. Have therefore kept
the forecast dry for Tuesday. Highs should be a bit below
normal...especially over west central WI where the lower clouds may
cut into diurnal heating. Expect readings to range from the lower
70s in west central WI to the middle and upper 70s in central and south
Long term...(tuesday night through monday)
issued at 258 CDT Monday Jun 29 2015
The large scale flow looks to deamplify a bit over the longer term
portion of the forecast... with the western ridge flattening and
retrograding... which leads to a fairly complicated split flow
from Alaska southward. The guidance differs markedly in its
handling of this large scale pattern evolution... with the gefs
and European model (ecmwf) ensembles also exhibiting quite a bit of spread. So...
confidence is a bit low in some of the forecast details from the
weekend into Monday as the guidance diverges quite a bit by then.
Chance probability of precipitation look to slowly work in from the west late Tuesday
night into Wednesday as the baroclinic zone on the eastern edge of
the upper ridge tries to slide east while a weak shortwave drops
southeast along it. Chances look to remain only over the western
portion of the forecast area... and shift back westward by
Wednesday night as the upper wave departs. Bring chances for precipitation
farther east across the area later Thursday and Thursday night...
with the European model (ecmwf) shortwave a weak surface trough and upper
shortwave moving across the area. The GFS maintains surface
ridging over the area during that time... so kept probability of precipitation low for
now. Lingered a low chance of precipitation into Friday as suggested by the
European model (ecmwf)... but allowed for a dry Saturday at this point. It/S
possible later forecasts may need to include something for
Saturday depending on how quickly return flow sets up and how
robust it is. For now... it looks like chances associated with
that will hold off until later Saturday night... and mainly only
impact the western portion of the forecast area... with a betters
shot of rain showers/thunderstorms and rain over the remainder of the area on Sunday with a
more significant upper wave and surface front forecast to move
across the area. The GFS and European model (ecmwf) both indicate this feature...
but the European model (ecmwf) is significantly stronger with the upper wave and
associated low level baroclinic zone. Lingered some chance probability of precipitation
into Monday... which allows for the weaker GFS solution that
lingers the baroclinic zone nearby and actually starts to lift it
back north as a warm front by Monday evening.
Aviation...(for the 00z tafs through 00z Tuesday evening)
issued at 606 PM CDT Monday Jun 29 2015
Showers and thunderstorms will affect the eastern half of the taf
area /kmsp-krnh-Keau/ until 03z...and then clearing skies are
expected with most areas remaining VFR overnight. Could see some
patchy fog overnight...but for the most part winds will stay up enough
to minimize that threat. Still expect Keau and krnh will see low-
end MVFR ceilings /1000-1500ft/ around/after daybreak Tuesday...as
low level moisture feeds in from the northeast. That layer will erode
by late morning...with a scattered-broken cumulus field developing by 18z.
In the wake of the thunderstorms...winds should settle in to a
northwest direction for the overnight hours...then shift to
northeasterly for Tuesday. Speeds at or below 8 kts.
another line of thunderstorms will pivot through east central Minnesota
between 00z and 02z. The strongest part of the line is expected to
scrape the eastern fringes of the field...with a reduction to
around 2sm and 2500ft the most likely impact. Wind gusts to 30 kts
are also possible. Skies clear after sunset with cumulus development
/Outlook for kmsp/
Wednesday...VFR. Winds east-northeast at 5-10 kts.
Thursday...VFR with chance MVFR -tsra. Winds southeast at 5-10 kts.
Friday...VFR with slight chance MVFR -tsra. Winds west-southwest 5-10 kts.