Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Hastings NE
313 PM CST Thursday Dec 18 2014
Short term...(this evening through friday)
issued at 300 PM CST Thursday Dec 18 2014
Drier air has begun to move to in aloft across the region this
afternoon behind the departing disturbance which brought some light
snow to the region early this morning. With this drier and warmer
air moving in at middle levels...the surface inversion has only
strengthened the past few hours...and little to no mixing has
occurred...preventing the low level stratus deck from eroding. As
a result...overcast skies have once again been the rule...with
temperatures struggling to climb into the upper 20s and lower 30s
across the local area. With very light winds in place and no
significant change in the general airmass...see no reason why this
stratus deck will not continue to prevail through the short term
periods. Consequently...expect the insulating cloud cover to help
hold temperatures up a few degrees overnight...and once again keep
afternoon temperatures a little bit cooler than they otherwise would be
Despite these cool and cloudy conditions...generally light southerly
winds will help make it feel a little more pleasant than it otherwise
could feel...as the pressure gradient near the surface will remain
weak with a broad area of surface high pressure across the
Midwest and a broad area of low pressure across The Rockies. All
this said...the main question through the short term periods lies
around the potential for some light freezing drizzle or fog
overnight through early Friday morning...as otherwise persistence
is expected for both temperatures and winds through the period.
Given the modeled soundings indicating somewhat favorable
conditions for the formation of drizzle combined with modest bulk
shear values forecast in the lowest layer of the atmosphere...had
a difficult time removing the mention of freezing drizzle from the
forecast despite its apparent lack of its appearance under similar
conditions the past few days. That said...the quantitative precipitation forecast fields from the
NAM this afternoon as well as the raw values of bulk shear are a
little more favorable than the past two days...so kept the small
potential in the forecast despite its seemingly low probability.
Did...however...fine tune the timing and location a bit per latest
Otherwise...there is also a modest potential for some fog formation
across the western fringes of the area overnight...as this area will
be closer to the edge of the low level stratus to our west. Both
rap/hrrr guidance suggest the western edges of our forecast area
at least have the potential to see some patchy locally dense fog
towards early Friday morning...so opted to keep mention of patchy
fog in the forecast...with no mention of any dense fog as think
the edge of the stratus will remain further west than currently
Long term...(friday night through thursday)
issued at 300 PM CST Thursday Dec 18 2014
Pattern: this forecast maintains excellent continuity with 24 hours ago.
The UKMET/Gem/GFS/ec and the Gem/GFS/ec ensemble means continue in
unison on the longwave flow. Amplification will occur over the Pacific
the next several days and downstream into North America. A longwave trough
will evolve near the dateline...and this will force a piece of the
subtropical high to break off and setup SW of California. This will raise
heights along the West Coast and result in a longwave trough over the
A potential forecast problem is the magnitude of the height falls over
the western USA Christmas day. The past 3 runs of the ec /prior to its
12z run/ and its ensemble mean are much deeper while the GFS/Gem are
flatter...a diff of 250-300 meters. This could be GFS/ec both
exhibiting their biases. This will need to be watched because it
will determine the strength of Lee cyclogenesis just after
Christmas. For what its Worth...the 12z ec came in flatter like the
GFS but probably is a one-run aberration.
Aloft: low amplitude flow will remain overhead through sun with some
inconsequential ripples passing through...and slowly rising heights
along the West Coast. Sun a strong shortwave trough will crest the ridge
in the exit region of a 175 knots anticyclonically-curve jet maximum. This
trough will dive southeast into the central/northern plains and close off Monday. Northwest
flow follows in its wake Tue-Wed. Heights rise Wednesday with a strong
shortwave trough digging over the western USA. The depth/strength will
determine our sensible weather Christmas day...ie a slower or faster
solution. Also of importance is where will the leading edge of the
colder air be? This will determine where Lee cyclogenesis occurs.
Surface: a warm front will form Sat-sun as low pressure organizes over
Alberta. The low will dive southeast into northern plains Sun night and drift
into Iowa Monday before heading into the Great Lakes Tuesday. Weak high pressure
will spread over the plains Wednesday. A warm front will form over the Southern
Plains Christmas with Lee cyclogenesis ensuing.
The daily details...
Friday night: difficult to know what happens with the stratus.
Ec/GFS/NAM cross sections suggest it will decrease...but a warm
front is forecast to develop and stratus would be supported on the
stable/cool side. Fog could be a problem near the edge of the
stratus. The sky forecast is troublesome and could result in large
errors in temperatures...which hinge on skies clearing west of Highway 281. Used
bias corrected met guidance which really dropped temperatures out west.
Sat: all depends on where the stratus is. Cloudy vs sunny. Cold vs
warm. Confidence very low.
Sun: probably mostly cloudy with multi-layered middle-high clouds...
if stratus is not a lingering issue. A few sprinkles possible over S-
Sun night: the Lee trough heads east across the forecast area as the low
dives into Minnesota. This could produce some sprinkles or a couple light
Mon: cold frontal passage. Possibly a few wrap around shwrs?
Tue: cold sector. Not looking quite as windy as 24 hours because how
the models are handling the strength of the Great Lakes low.
Wed: am not buying the GFS taking a clipper to the north of the forecast
area...but the ec sneaking something down the Front Range looks from
plausible and this could bring a touch of light snow to parts of the
region. Lots of uncertainty...so this is not a lock.
Christmas day: Lee cyclogenesis with a warm front forming/
strengthening. Expect a warm air advection pattern to develop...but timing remains
uncertain and location of the low/front will be key in subsequent
sensible weather. Suggest all with travel plans keep an eye on the forecast.
Models and ensembles continue to indicate a transition back to the
pattern of Nov late month...with a ridge over Alaska and Arctic chill
returning. It will be turning colder after Christmas.
Aviation...(for the 18z kgri/kear tafs through 18z Friday afternoon)
issued at 1149 am CST Thursday Dec 18 2014
Generally poor conditions will continue at both terminals for the
next 24 hours. While there has been some improvement in ceilings to
MVFR levels this afternoon...expect this improvement to be short
lived...with ceilings dropping by down to IFR levels over the next
couple of hours. With very light winds...the timing of this is
difficult...but think IFR ceilings will likely return by 18/22z...with
further deteriorating ceilings along with some br or even dz possible
during the overnight hours.