Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Fort Worth Texas
556 PM CST Friday Dec 26 2014

Aviation...
strong/moist southerly flow resulted in low level saturation
earlier today but the low ceilings did break for western suite of taf
sites. This has mixed out some of the higher moisture for these
sites and VFR currently prevails. Still IFR ceilings remain across
eastern Dallas County...a sign that MVFR/IFR ceilings should build
in from east to west during the evening hours. Dal/dfw/gky should
all be back to MVFR by 1z and go IFR around 4z. Ftw/afw/act should
stay VFR until 2-3z...with ceilings not falling to IFR until after 6z.

Cold front will arrive in the metroplex between 8-9z...switching
the light south winds to north/northwest at 10-15kt. IFR ceilings and some
drizzle reducing visibility to MVFR is possible along and just behind
the front. However deepening cold/dry airmass will cause ceilings to
lift to low MVFR by 10z with continued improvement to above 2000ft
by middle morning Saturday. Ceilings should lift to VFR by midday
Saturday. Meanwhile the north winds will increase to 15g25kt by
daybreak. For act...frontal passage will occur around 10z...with MVFR ceilings
prevailing all day Saturday.

Good elevated moisture and weak instability will be present near
700mb as best lift moves across the region Saturday afternoon and
evening. This should result in scattered showers over the
area...but generally no aviation impacts are expected from this
activity and have just mentioned vcsh in tafs.

Tr.92

&&



Previous discussion... /issued 359 PM CST Friday Dec 26 2014/

Early afternoon water vapor satellite imagery showed a large upper
trough over the Continental U.S. Rockies with broad ridging over the eastern
half of the Continental U.S. And an amplified ridge centered over the Yukon
and extending south over the northwestern Continental U.S. Coast. A strong
shortwave trough was observed over the far northeastern Pacific
Ocean...south of Anchorage Alaska. Locally...visible satellite
imagery showed a thick deck of low clouds over locations along and
east of the Interstate 35/35w corridor. Just to the west of the
Interstate 35 corridor...skies were mostly clear. This relatively
sharp edge to the cloud cover in place across north and central
Texas resulted in a fairly strong west to east temperature
gradient across the County Warning Area this afternoon. The temperature at dfw at
230 PM was 60 while the temperature at Granbury was 71. Regional
radars and surface observations continue to indicate some pockets
of very light rain/sprinkles moving north within the deck of
clouds east of Interstate 35 this afternoon.

This light rain is expected to persist...but will not amount to
much more than a hundredth of an inch of rain through this
evening. Between cloud cover and southerly winds of 10 to 15
miles per hour...temperatures are expected to remain mild across much of the
County Warning Area until late this evening. A piece of energy aloft embedded with
the Continental U.S. Rockies upper trough is expected to eject northeast over
the Southern Plains this evening. This is expected to send the
nearly stationary cold front...currently extending from southwest
Oklahoma across the southern Texas Panhandle...southeast across
north central Texas tonight. This front is actually fairly
strong...and has a seasonably cold airmass behind it...so it is
expected to move across the County Warning Area fairly quickly tonight.
Temperatures will drop 10 to 15 degrees pretty quickly behind the
front...resulting in a much cooler start to the day tomorrow
across much of the region.

Isolated to scattered rain showers are possible along the
front...but rain chances are expected to be slightly higher behind
the front tomorrow afternoon. The front is expected to move
southeast towards the Gulf Coast shortly after sunrise...resulting
in temperatures that are steady or possibly slowly falling
throughout the day. With the front in place...the last piece of
strong energy aloft within the larger Continental U.S. Rockies upper trough
is expected to move east over Texas tomorrow. The large scale
forcing for ascent induced by this trough will likely bring the
best rain chances to the region tomorrow afternoon. The best rain
chances seem to exist southeast of a 700 mb baroclinic zone/front
tomorrow. This middle-level front is expected to be located near a
line from Bonham to Fort Worth to Comanche tomorrow afternoon...so
went ahead with 30-50 probability of precipitation southeast of this line...and 20 probability of precipitation to
the northwest. The forcing for ascent does not look overly strong
at this time...however a cross sectional analysis of model mass
fields indicates that there is a small area of conditional
symmetric (csi) to upright instability present just above the
sloping 850-700 mb front tomorrow. From a forcing stand point
alone...the probability of precipitation are probably too high as coverage would likely be
isolated at best. However with the potential for the release of
cape or csi over the area...went with the higher rain chances
assuming the weak lift would result in some decent displacement
due to the stability considerations mentioned above. Rainfall
amounts will likely remain less than one tenth of an inch on
average. Any thunderstorms that are able to develop would result
in very localized higher amounts.

Precipitation is expected to come to an end from northwest to
southeast Saturday night as the upper level energy moves east
spreading large scale forcing for ascent over the region. Clouds
will clear out from west to east during the day on
Sunday...resulting in mostly clear skies over the region by late
Sunday afternoon. Kept sunday's highs a bit on the cool side as
morning cloud cover...and a late afternoon shift to south winds
will prevent a quick warm up from the cool airmass that builds
over the region behind tonight's front.

The strong shortwave trough south of Anchorage this afternoon is
expected to ride up and over the Yukon upper ridge this
weekend...and then dig almost due south over the western Continental U.S.
Coast early next week. With strong ridging over western
Canada...and a seasonal...deep...somewhat persistent...upper low
over the Hudson Bay...this upper trough diving south early next
week is expected to send a strong Arctic airmass south into the
plains. All guidance indicates that this Arctic airmass will begin
surging south on Monday...moving over the Southern Plains Monday
night into Tuesday morning. But...before the Arctic airmass
arrives...sunny skies and southwesterly winds will likely result
in a seasonably warm and pleasant day across North Texas on
Monday.

Regarding the Arctic airmass...it is expected to move across all
of North Texas by Tuesday afternoon...however it may move in
quicker than that if the 12 and 18z NAM model runs verify. There
is little doubt that a strong Arctic airmass will surge south
along the High Plains on Monday...the main questions for north
central Texas are...how fast will it get here? And how cold will
it get? Medium range models notoriously struggle with the speed of
Arctic air moving south across the High Plains. As a
result...think that the faster NAM solution is very likely to
verify even though it does not represent the consensus of model
guidance very well. As a result...went ahead with a cooler
solution regarding high temperatures across north and central
Texas on Tuesday. Expect strong cold air advection to be ongoing
across the entire County Warning Area by the time the sun rises Tuesday morning.

How cold will it get? This represents the more complicated
question to evaluate as there are multiple ways that Arctic air
can be modified as it makes its 1500 mile trek south towards North
Texas. The most efficient ways that the brunt of the Arctic air is
modified or warmed is through downslope winds...and just basic
heating by the sun. Along the entire 1500 mile trek south...the
Arctic airmass will ride along the Lee of The Rockies...and any
westerly winds that impinge upon this airmass could help to
slightly warm the air...reducing the strength of the cold air as
it continues south. Also...the farther the air sinks south...the
greater the sun angle acting upon the Arctic airmass...allowing
the sun to simply warm up the air the farther south it moves.

One of the best ways to evaluate the the potential for the Arctic
air to warm up as it heads south is to look at model middle-level
wind fields. Westerly middle-level flow over The Rockies will induce
downslope winds within the Arctic airmass. Westerly winds are also
more likely to clear out middle to upper level clouds as the basic
response to middle-level air moving over the mountains/higher terrain
is to sink. Westerly winds do not guarantee clear skies within the
Arctic air...but it makes it more likely to get some sunshine on
the cold air. At this time...the consensus of guidance indicates
that middle-level flow will be very weak...or very weak but
easterly...as the Arctic air moves south along the plains Monday
and Tuesday. Easterly middle-level flow would seem to provide optimal
conditions for the airmass to build south without warming up as
this would result in enhanced cold air damming along the High
Plains...and would actually support more clouds over the cold
airmass.

With seemingly ideal middle-level flow over the Arctic airmass Monday
and Tuesday...confidence in a strong Arctic airmass building over
north central Texas is high. Went ahead and bumped temperatures
down several degrees from the previous forecast as a result. Raw
model 2-meter temperature fields are likely several degrees to
warm across the region once the Arctic air is in place...but that
is very difficult to prove at this point. GFS and European model (ecmwf) 2-meter
temperatures have been too warm with regards to forecasting most
Arctic outbreaks for north central Texas from personal experience.
That does not mean they are too warm this time...however pattern
recognition and past experiences strongly suggest that the region
will be colder than what models are advertising at this time. As a
result...have forecast highs and lows from Tuesday through
Thursday night that are on the cold side of all available
guidance. It is possible that this is not quite cold
enough...however cannot reasonably forecast temperatures colder
than what any model shows at this time.

There is also ensemble support for siding with the colder side of
all guidance in this forecast. The naefs advertises a nearly plus
3 Standard deviation for mean sea level pressure over the South
Plains on Wednesday and Thursday on the naefs standardized anomaly
Page. Also...every member of the 12z GFS ensemble system indicated
a -3 to -10 degree c 2-meter temperature anomaly over north central
Texas on New Years Day. Felt that this was enough support to side
with colder temperatures in this forecast.

With high confidence that cold air will be in place across north
central Texas Tuesday through Thursday...the attention in the
forecast turns to precipitation chances.

Unfortunately 12z model guidance does not offer much in the way of
raising confidence in the precipitation forecast for next week.
Our precipitation chances are tied to the timing of the Anchorage
shortwave trough moving east over the region next week. 12z
guidance is faster with this trough...however as we've seen the
past couple of days...models are changing the timing of this upper
trough by 24 to 30 hours per model run. This represents the
potential for large errors in any deterministic forecast for
precipitation until models can start to agree on timing better.

In short...the faster the upper trough moves over North
Texas...the greater our chances of wintry precipitation next week.

Models are struggling so much with the timing of this upper trough
because nearly all guidance indicates that the upper trough will
cut off into a closed low by new year's evening. The farther removed
from the polar jet stream this closed low gets...the slower it
will tend to move over the region. With the upper trough closed
off...there will be no reinforcing of the Arctic airmass over the
plains for late next week. This will allow the Arctic airmass to
begin to modify/warm up quickly...likely not posing a threat for
wintry precipitation any more by January 2nd. So...just like
yesterday the forecast for wintry precipitation remains uncertain
at this time.

The forecast confidence remains similar to yesterday. Confidence
in low-level air being cold enough for wintry precipitation/ice is
highest on Tuesday and Wednesday...and lowest by January 2nd where
we will probably be too warm for any wintry precipitation.
Confidence in precipitation/lift is lowest on Tuesday and
Wednesday...and increases quite a bit on January 2nd...when most
12z guidance moves the upper trough over North Texas. So it is
where the confidence in cold air is moderate...and confidence in
precipitation is increasing...Thursday...where our chances for
wintry precipitation appear to be maximized at this time.

There's Little Point getting into precipitation type forecasting
this far out as precipitation type is usually determined by
weather elements not resolved until 48 hours before a wintry
event. However...with a shallow Arctic airmass in place...and warm
air advection above it mid-week...the large scale pattern supports
freezing rain more-so than sleet or snow at this time. Went ahead
and placed a mention of rain or freezing rain back in the forecast
for much of the region on Wednesday and Thursday as a result of
all that has been discussed regarding this Arctic airmass so far.
It remains entirely possible that the upper trough will simply be
too slow and we will just remain cold and dry across North Texas.
While that is possible...there are too many wetter solutions that
indicate ice is a real threat to leave it out of the forecast
entirely at this time.

Most guidance indicates that we will warm above freezing by Friday
January 2nd...with widespread rain showers likely assuming the
upper trough moves over the region at that time.

Cavanaugh

&&




Preliminary point temps/pops...
Dallas-ft. Worth, Texas 45 46 32 49 34 / 30 30 10 10 5
Waco, Texas 49 50 34 51 32 / 30 30 20 10 5
Paris, Texas 50 51 33 48 31 / 50 40 10 10 5
Denton, Texas 41 45 29 49 31 / 30 20 10 10 5
McKinney, Texas 45 46 31 48 30 / 40 30 10 10 5
Dallas, Texas 46 47 33 49 35 / 30 30 10 10 5
Terrell, Texas 52 53 34 49 32 / 40 40 10 10 5
Corsicana, Texas 54 55 35 49 33 / 40 40 20 10 5
Temple, Texas 51 52 34 51 32 / 30 40 20 10 5
Mineral Wells, Texas 38 44 28 49 31 / 20 20 10 5 5

&&

Forward watches/warnings/advisories...
none.

&&

$$

/

National Weather Service Glossary of Abbreviations