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Area forecast discussion...updated 
National Weather Service Jackson Kentucky
737 am EDT sun Mar 29 2015

Update...
issued at 700 am EDT sun Mar 29 2015

Forecast is on track this morning. Kentucky mesonet indicating
many locations have dropped into the middle and upper teens this
morning. We have seen a record low temperature here at jkl as well.
Temperatures will warm quickly after sunrise. Forecast package only
required a few tweaks to bring grids in line with hourly observation.

&&

Short term...(today through monday)
issued at 400 am EDT sun Mar 29 2015

Two issues to deal with...first being very low relative humidities
this afternoon. Second concerns the passage of a surface cold frontal
system tonight.

Short wave ridging aloft and surface high pressure moving off to the
east through the day will keep skies mostly clear through much of
the day as southerly return flow increases. Combined this will
allow very dry air to mix down into the boundary layer and
eventually to the surface...all while temperatures make a strong
rebound. As a result relative humidities will drop into the teens
by this afternoon...well below critical fire weather thresholds.

Short wave trough moving through the Great Lakes late today and
tonight will send a surface cold frontal boundary through the region.
Clouds will increase ahead of this system late this afternoon.
Models also develop a band of showers along and ahead of this
system. However...very dry boundary layer will eat away at some of
the model/S advertised quantitative precipitation forecast values. Consequently we are expecting
one of those relatively high pop...low quantitative precipitation forecast events. The frontal
boundary will be through eastern Kentucky by Monday morning.
Fortunately...considering that the origin of the air mass behind
this front is Pacific in nature...temperatures will turn out to be
warmer behind the system than ahead. Stayed fairly close to the
blend of model solutions and guidance for probability of precipitation and temperatures.

Long term...(monday night through saturday)
issued at 440 am EDT sun Mar 29 2015

The recent GFS and European model (ecmwf) runs are generally good agreement trough
the period. The period is expected to begin with a western US ridge
extending north into the Canadian rockies and eastern Alaska...with a
trough from the plains into eastern Canada and the eastern US.
The axis of this trough is expected to extend from northern Canada
south into the northeastern states and then south to the east of
Florida. An upper level low is projected to be to the west of the Baja California
of California at that time as will a trough extending from the
Gulf of Alaska south across portions of the eastern Pacific to the
west of the US and Canadian coast.

As the week progresses the pattern will have a zonal flavor with the
trough over eastern Canada and northeastern states becoming more
broad and the ridge initially over the West Coast shifting to the
plains by midweek. Meanwhile...the trough initially west of the Baja California
of California is expected to get kicked into the Southern Plains
by the trough working into the western US. This by Wednesday
evening...the 0z GFS and 0z consensus is for ridge axis to have
moved to the lower Ohio Valley to Great Lakes with some model
variability with the shortwave moving from the Southern Plains
into the MS valley region. At that point...heights are expected to
be falling over the plains region. Wednesday night into Thursday...the
shortwave ridge axis is projected to have shifted further east and
northeast from the Middle Atlantic States north into Ontario and
Quebec with uncertainty increasing in the strength of the trough
moving at that point from the plains toward the MS valley region
and western Great Lakes. The model consensus from the 0z GFS and
0z European model (ecmwf) is for the shortwave trough to generally pass south of
eastern Kentucky across the southeastern states...but upper level flow
becoming more westerly to southwesterly by late Thursday into Thursday night
as the 500 mb trough approaches leading to a warmer and more moist
air mass moving in. The consensus is also for the trough to move
further east into Ontario and Quebec as well as the Great Lakes
and near the Ohio Valley region by the end of the period. Just
beyond the period...a ridge is expected across The Rockies with
eastern Canadian to eastern Continental U.S. Troughing.

This pattern will bring at least somewhat changeable weather due to
the quasi zonal pattern. Early in the period...weak surface high
pressure is expected to be centered over the middle Tennessee to central Kentucky
region...which moves into the southeastern states on Monday night into
Tuesday. Meanwhile a northern stream cold front should drop south of the
Great Lakes and into the Ohio Valley region.On Tuesday into Tuesday
night...but be rather moisture starved at that point with high
pressure over the Great Lakes and northeast nosing into the area for
a time. Meanwhile...as a low pressure system develops and tracks
rather quickly near the US Canadian border to just north of Lake
Superior from Wednesday into Wednesday night...a warm front should lift north
across eastern Kentucky. This will put the region in the warm sector for
Thursday into Friday...with anther wave developing along the front and
tracking from the plains to middle MS valley region Thursday into early
Friday and then further northeast which may slow down the eastward
progress of the cold front associated as the trough gradually moves
into the eastern Continental U.S. Around the end of the period. During this
period...from late Wednesday night into Saturday there will be a chance
for showers. The chances will be highest from Thursday night into
Friday night. Model guidance suggests there could be two periods
where precipitation chances possibly maximum out...from Thursday night into
early on Friday and then late Friday into Friday evening. The first
round would be with some insentropic lift and possibly some jet
interaction and the second round near and after the cold frontal
passage and possible interaction with some upper jet dynamics.
Moisture increases such that thunder has been included from Thursday
into Friday...but chances are probably best Thursday night as winds
through the column increase and with precipitable water increasing some good rainers
and possibly a strong storm or two. Chances may also peak at some
point on Friday...pending the timing of the frontal passage.

There should be a gradual moderating trend from Tuesday into Thursday or Friday
as 850 mb temperatures climb from not far from 0c at present to 10 to 12c
by Thursday into Friday. A drop in 850 mb temperatures should drop as the trough
moves in late next week with the current model consensus only down
to around -5c...but there is uncertainty and time will tell.

&&

Aviation...(for the 12z tafs through 12z Monday morning)
issued at 735 am EDT sun Mar 29 2015

Surface high pressure gradually departing the area will keep skies
clear and winds light through around 14z...before the nocturnal
inversion breaks. As this high pressure moves further east southerly
gradient flow will increase through the day...ahead of an approaching
low pressure system. Winds should increase to 5 to 10kt and
become around 14z...especially at sme and sym. Clouds will
increase and isolated to scattered showers will begin to develop
0z to 3z...with an area of showers expected to move across the
area as the low pressure system works through. Visible and or ceilings
should drop into the MVFR range as the area of showers moves
through.

&&

Jkl watches/warnings/advisories...
none.
&&

$$

Update...ray
short term...ray
long term...jp
aviation...ray/jp

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