Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Twin Cities/Chanhassen Minnesota
953 PM CDT Wednesday Aug 27 2014
issued at 953 PM CDT Wednesday Aug 27 2014
Grids were updated earlier to reflect a slowing trend of the
Central Plains complex reaching Minnesota Thursday morning. A deep dry
layer in place will take a bit to saturate late tonight so kept
the forecast dry for most areas north of the Minnesota river. Water vapor
imagery shows the complex forming on the nose of the middle level dry
slot over northestern Colorado and southwestern NE. Models have stabilized somewhat in
their northward and southward shifting although the specific
details still need to be worked out following the rest of the 00z
model suite and as the timeframe begins to enter the hi res model
Short term...(this evening through thursday)
issued at 316 PM CDT Wednesday Aug 27 2014
The primary concern in the short term period revolves around
precipitation onset and amounts with the trough that is ejecting
northeast from The Four Corners region over the next 24 hours.
Model solutions have trended slower with the timing of this
system...and it now looks to be mainly after 06z before showers
and thunderstorms arrive into the southwestern forecast area...and
not until closer to Thursday afternoon for locations along/north
of the Interstate 94 corridor. Regarding precipitation amounts...a rather
disturbing shift in the highest totals was witnessed with the
27.12z GFS...which developed a lead shortwave trough and secondary
maxima that lifted across west central and central Minnesota to Lake
Superior. This second area was in addition to the southern maxima
along the Minnesota/Iowa border in closer proximity to the baroclinic zone.
This scenario depicted by the GFS looks fishy...and has been
discounted this forecast cycle. Went with a consensus blend that
includes a heavier weight toward the 27.12z NAM and European model (ecmwf) models.
While there will likely be shortwave energy that instigates
broader precipitation north of the heavier southern band...it
seems more logical that the attendant amounts would be lighter
than what the GFS depicts. Between 06z Thursday and 00z
Friday...precipitation amounts in the 1 to 2 inch range look likely over
southern Minnesota...tapering down to around one half inch along
the Interstate 94 corridor...and under one half inch to the north.
High temperatures on Thursday are expected to top out in the
low/middle 70s under the extensive clouds and rain.
Long term...(thursday night through wednesday)
issued at 316 PM CDT Wednesday Aug 27 2014
The rain producer that will be moving through the area has slowed
with recent models trends as the upper low moving out of the 4
corners struggles through the nations mid-section. The trough
never quite takes on a negative tilt...but with downstream ridging
over the eastern Continental U.S....the progression will be slow. With that
said...the Thursday night and Friday periods will still be susceptible to
rain and embedded ts with continued moisture transport from the
Gulf...thus probability of precipitation have been raised accordingly. Some heavy rain is
still a threat with severe probabilities fairly low under the
moist column. A northern stream shortwave moving into the Dakotas
by late Friday will help to finally kick this system east for the
weekend which will end up being fairly dry. Another trough
originating from the pacnw will spread -shra/ts chances from west
to east across the upper Midwest on sun...starting out in western
Minnesota as early as late morning...mainly evening/overnight from the
Twin Cities east. This may also be moisture laden (aka heavy rain)
as moisture from the Gulf gets drawn northward once again.
Temperatures over the weekend are expected to be a few degrees
above normal...especially if we can get some breaks in the cloud
cover. The upper low north of the international border and
associated trough axis may be slow to push through early next week
which may trigger a few showers or isolated T...but with northwest flow
developing in its wake...any precipitation should be diurnally driven
with temperatures trending slightly below normal.
Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 708 PM CDT Wednesday Aug 27 2014
VFR conditions this evening and early overnight...then some light
fog may develop for a time at stc/rnh/eau near dawn Thursday.
However...increasing middle and high level clouds may hamper this
altogether. A surge of moisture will push northeast Thursday with
clouds lowering and precipitation becoming widespread across southern and
eastern Minnesota and western Wisconsin through the day. MVFR
conditions and brief instances of IFR with thunder can be expected
by Thursday afternoon.
Kmsp...rain looks to arrive sometime about midday with conditions
deteriorating thereafter. In the mean time...VFR conditions are
/Outlook for kmsp/
Friday...VFR. Slight chance of MVFR/tsra. Wind west-northwest at 5-7kts.
Sat...VFR. Wind north-northeast at 5-10 kts.
Sun...VFR with MVFR/thunderstorms and rain late. Winds south-southeast at 10kts.