"But seeing they could not See; hearing they could not Hear"
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"From its chamber comes the whirlwind, and cold from the scattering winds." - Job 37:9.

"The wind blows to the south and turns to the north; round and round it goes, ever returning on its course".

Wednesday, December 22, 2010

"A Nation Divided" This Afternoon - Forecast Havoc East of the Mississippi

Images: Included is a forecast map for around midnight showing a 'cold front'/wind shift line over or near Central Florida as a full latitudinal blocking ridge axis extends across the nations midsection from near the Arctic Ocean to the Gulf of Mexico. See below for why this is so problematic for the weather forecast in the next few days. I've included a map of Canada and the Arctic Ocean for kicks.

REMAINDER OF WEDNESDAY: Widespread mid-70s degree temperature readings have overspread much of the Florida peninsula this afternoon with steady 'generally' westerly winds prevailing around 15mph in most areas. The coolest spots are along the immediate west coast close to the Gulf waters and, of all places, the Florida Keys where temeratures are in the upper 60s. Skies would be considered partly cloudy in most if not all locales as well.

As we can see by the graphic included, a cold front which now extends across Central portions of Georgia and Alabama will move into Central Florida around midnight. There is little temperature variation associated with this boundary in looking at the latest surface observations, and as such I'm looking at this 'front' as somewhat as reassurance that high pressure stretching from the far reaches of Northern Canada to nearly the Gulf of Mexico is in full control of the Mississippi River Valley basin. This high pressure is going to be the thorn in our side from a forecaster's perspective everywhere east of the Mississippi River as we head into Christmas Day through two days after. See further down in this post for details.

In the meantime, I've annotated a very rough approximation of the front's location shortly after midnight tonight. As we can see, it will be located somewhere over Central portions of the peninsula and by morning will have nearly, if not totally, cleared the state. Behind the front winds take on a more northwesterly component for Thursday and steadily become more northerly during the early portions of the day. By mid-late afternoon they should be NNE-NE and weaken in speed as the high pressure system reasserts full control of the weather locally.

The first fly in the ointment in today's post is cloud cover associated with the boundary. For now, I'd expect partly cloudy sky conditions all evening possibly becoming overcast around midnight and through the pre-dawn hours. Just how much cloud cover remains after day break is a question mark in my mind, especially along and east of I-95 from Daytona to Miami. For now, we'll ride with partly cloudy skies, but a period of 'cloudy' could occur along the barrier islands region through early-mid morning Thursday at least. No rain though. Cloud cover will keep temperatures from falling tonight, mainly 50s along the coast and some 40s well inland. We will need to watch those clouds tomorrow from the Cape to Miami though, since it is not a total given we will see more sun than clouds Thursday.

THURSDAY: Winds from the NW from 10-18mph initially will gradually veer to NE by days end and weaken during the course of the day. Now behind the front, temperatures will remain in the mid-60s with the final outcome contingent on the amount (if any) of cloud cover closer to the coast as noted in the preceding paragraph. Either way, cloud cover or shifting, slightly onshore gradient winds will keep the high temperature in the mid 60s range with lows again dropping into the low to mid 50s overnight with 40s inland and further north..

FRIDAY-CHRISTMAS EVE: Light onshore flow and high pressure nearly overhead will provide for another day of partly cloudy skies and another cool day, yet still comfortable with temperatures comparable to Thursday or perhaps a little warmer with less chance of cloud coverage initially. I've been secretly hoping for a reason for the immediate coast to see some afternoon sea-fog to come in on Christmas Eve. I've NEVER seen such events forecast before hand as they are very rare, maybe every 10 years or so from my experience. There was one such event just two years ago, so we aren't due yet...but maybe Santa can grant this one simple wish.

At this points is where I'd like to interject. Energy associated with the weather system generating the rains over Southern California will have moved east, much of which will try to 'divert' the blocking ridge of high pressure by hedging across S. Texas. Thus, instead of taking a more NE-ENE trek it will ride east to ESE across Texas. Not all of the energy, but some of it. Exactly how much is totally uncertain. Temperatures yesterday and today have been very warm over extreme South East Texas (not so much today). But regardless, this heat will provide the ample fuel necessary to instigate surface low cyclogenesis over the far Western Gulf by late Christmas Eve through Christmas Day. The surface low will continue to develop as it tracks east across the north central Gulf Chrismas Day.

Meanwhile, in viewing the forecast mid level winds for Christmas Day, believe a lot of mid-upper level clouds could eject eastward well in advance of this developing scenario that will spread across all of Florida over night or beginning sometime Christmas Day. Temperatures will remain moderated overnight and during the day Christmas as a result.

CHRISTMAS DAY: No frontal passage as previously presumed. Models have delayed the front's arrival by a full 12-18 hours, and if the trend continues, could be delayed another 6 hours. Good for Florida initially. This means, Christmas Day now looks to be rain free; but again, I think we'll see a lot of clouds keeping high temperatures in the 60s rather than the advertised 70s I'm seeing on TV. Winds will initially by southerly and light with the dawn of day, but picking up increasingly from the SSW-SW during the day.

CHRISTMAS NIGHT-BEYOND: Instead of getting into the nitty-gritty details at this point for this time frame, it is best to leave this time frame open for what might be coming down the pipe...a lot will be contingent upon 1) the track the surface low takes as it approaches the state; and 2) strength of the surface low (not to mention the other technical variables involved).

As it stands, we could be seeing reason for thunderstorm development (some severe) by late overnight Christmas Night toward day break the day after Christmas. At this point, the surface low looks to cross North Central or Central Florida shortly after sunrise Sunday...but my gut says it may end up crossing even further south with greater intensity. If so, the threat for severe weather, especially for South Central Florida will be even greater. Either way, the chance for rain at this point looks pretty high sometime after midnight Christmas night through mid-afternoon the day after Christmas (Sunday).

This will all be refined once the storm system works its way across the Desert SW region...but even more so when we see the result of this system as it interacts with the 'hot spot' over Eastern and Southeast Texas over night Thursday through Friday and as the system morphs while crossing the Gulf of Mexico.

POST FRONTAL PASSAGE: Eventually, whatever track the low pressure system takes, it stands that the front will rapidly clear the state by Monday at the utmost latest followed by steady cold air advection from the Northwest all day Monday. And again, possibly as soon as early Sunday morning. Either way, cold air advection will be in place Monday and Tuesday making for two cold days. It does not look as if this cold period will be as chilly as our earlier counterpart last week at this time. But freezing temperatures as described in yesterday's post accompanied by a good dose of wind will likely result in issuance of wind chill advisories and warnings for Monday and possibly Tuesday under a 'freeze advection' type synoptic set up.

NEW YEARS: What, how can one come up with a forecast for New Year's Eve/Day when we already have our hands full not only with tomorrow, but with the days surrounding Christmas? Had to throw this in because I couldn't help myself -- strictly for amusement. That is, if the ECMWF and GFS hold fast, we could be in for a big WARM UP come New Year's Eve/Day time frame. Can it be? We'll see, but did want to throw that tasty tid bit out there for chomping on until this time frame draws closer. Consider it a stocking stuffer.

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