Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Twin Cities/Chanhassen Minnesota
708 PM CDT Tuesday Sep 16 2014
Short term...(this evening through wednesday)
issued at 214 PM CDT Tuesday Sep 16 2014
Once again a quiet short term is expected with the only minor
concern being the potential for ground fog in low lying areas and
perhaps a bit of frost in western WI. Overall...temperatures are
expected to be several degrees warmer tonight with the light
southerly flow. Winds should be very light late tonight...but
there is also the chance for a pretty good middle deck of clouds
across at least southern Minnesota with a shortwave expected to
pass through. I kept the patchy fog previously issued - like
keeping the fog to the coolest parts of the forecast area where
there is a better chance of clear skies or thin clouds. Pleasant
Long term...(wednesday night through tuesday)
issued at 214 PM CDT Tuesday Sep 16 2014
Large scale pattern evolution...
guidance continues to advertise a similar evolution over the past
several runs... with the western ridge flattening as it shifts
eastward into the central continent by the end of the week. After
that... there remains some spread in the details... but everything
still favors a re-amplification of the upper level pattern with
the western ridge/eastern trough pattern resuming by the end of
the weekend into the first half of next week.
Forecast item/S/ of concern...
primary item of concern is rain showers/thunderstorms and rain Friday into Saturday. Locally
heavy rainfall is still possible with thunderstorms... but
widespread heavy rain does not appear to be a concern given the
progressive nature of things. Some severe weather does look
possible along ahead of the cold front late Friday... although
timing will certainly play a role in the potential.
the deterministic and ensemble solutions continue to agree quite
well on the overall large scale evolution... with differences in
timing and amplitude of individual shortwaves... particularly
during the re-amplification of the pattern from Saturday into
Monday. The latest European model (ecmwf) has changed its tune once again... and is
much less amplified with the digging trough in the upper Midwest
and Great Lakes over the weekend. However... it still winds up
with a similar long wave pattern by the early part of the week...
and matches up reasonably well with the GFS and gefs. The gefs and
GFS are very similar with the long wave pattern... and gefs 500mb
height Standard deviations suggest that any error in the
deterministic guidance would be with it being too far east with the
ridge/trough axes from Sunday into Tuesday. In other words... the
spread in the ensemble guidance is mainly due to individual
members digging the trough a bit closer to US while keeping the
ridge farther west. Given the overall decent agreement on the big
picture... and continue disagreement on details... a fairly
evenly split consensus approach continues to be the favored course
Sensible weather expectations...
dry weather and seasonable temperatures will persist from
Wednesday night through Thursday as the surface high drifts to the
southeast and we slowly get into return flow. Precipitation chances look to
return from southwest to northeast Thursday night into Friday as
warm advection increases and we eventually see sufficient
moistening of the profile along with some convective instability
above 850mb. We should see that warm advection band of rain showers/thunderstorms and rain
lift northeast across the area on Friday as the cap builds
northeast into the area. We should then see additional rain showers/thunderstorms and rain
something late Friday into early Saturday as the cold front moves
across the area. Some precipitation will likely linger into Saturday across
the area as the upper trough pushes into the area. By Sunday it
looks like we/ll have sufficient subsidence and drying for things
to start to clear out... although the models have varied quite a
bit on the strength of the middle-upper level cold pool... which
could result in more diurnal cloud cover and rain showers than currently
advertised. High pressure then looks to settle in across the area
for Monday into Tuesday... with cool but dry conditions expected.
Detailed discussion for item/S/ of concern...
at this point... cold frontal passage looks to occur some time Friday night.
The NAM and GFS both show a Stout cap around 800mb... so it/S
tough to say whether we/ll see much convection develop too long
before the arrival of the middle-level shortwave and surface front.
Timing will certainly play a role on the potential for any severe
weather. During peak heating into early evening... MLCAPE values
at or above 1500 j/kg look to be a good bet... and the latest runs are
more bullish with respect to deep layer shear showing 0-6km bulk
shear at or above 45kt and 0-3k srh well above 400 m2/s2. Low level shear
appears to mainly favor wind vs strong low-level mesocyclones at
this point... so any potential severe threat would appear to be
mainly hail/wind. Will need to keep an eye on how things evolve...
but at this point there at least appears to be a marginal chance
for some severe storms late Friday.
confidence in the overall evolution of the large scale pattern and
timing of main precipitation window on Friday into early Saturday is fairly
high. Primary uncertainty is with any severe weather potential
late Friday... and the degree to which we cool off Sunday into
Aviation...(for the 00z tafs through 00z Wednesday evening)
issued at 637 PM CDT Tuesday Sep 16 2014
Some changes were made to the tafs for this issuance based on
satellite trends of increasing cloud cover over western Minnesota. Short
term guidance shows this cloud cover continuing to expand early
this evening and drift east overnight. All bases will remain VFR.
This also will limit the fog potential overnight...so removed the
mention near dawn at eau.
Kmsp...looking at increasing middle level clouds this evening that
should last the majority of the night. Decreasing clouds expected
/Outlook for kmsp/
Thursday...VFR. Winds southeast at 10-15 kts.
Friday...VFR likely. Slight chance MVFR with -shra. Winds S-SW
Sat...VFR. Winds northwest 10-15 knots.