Warmth and Showers Leading Up to Christmas

    Winter officially begins on December 21st, but it will feel a lot more like October over the next several days, as temperatures increase throughout the week, culminating in what will likely be record-shattering warmth on Christmas Eve.  Through December 19th, Philadelphia is 11.2° above average for the month, and above average temperatures will generally be the main story through the end of the year.  The period of December 10-18th saw temperatures of 11-24° above average itself, and no day this month has been below average thus far.  Below is my forecast for our holiday week, with still some uncertainty for rainfall totals on Wednesday and temperatures for Christmas Day.

Monday

  As a weak cold front approaches from the west, it will dissipate and 'likely' leave us dry.  We'll see a gradual increase in clouds through the day, with partly/mostly cloudy conditions in general.  Our week of mild temperatures will begin with a high of 55-60°.

Tuesday

  An upper-level disturbance moves through the Northeast and brings clouds and the threat of showers for the morning/afternoon, with any precipitation likely ending by sunset.  We'll likely see upward of a tenth of an inch or so, with a high of around 60°.

Wednesday

  A frontal boundary approaches from the west, a warm front will move through the area, and precipitation will overtake the area.  Periods of showers and rain will fall throughout the day as a low pressure system strengthens over the Great Lakes, providing for windy conditions as well as rain.  It appears at this time, with upward of 0.50-1.00" possible and a high in the mid-60's (Philadelphia's record high is 66°).  Below is modeled radar for 4pm on Wednesday:

Modeled radar from the NAM for 4pm on 12/23 (Image courtesy of TropicalTidbits.com)

Christmas Eve

  Any lingering showers end by the afternoon, and high pressure from the west should begin to dry things out with some clearing.  We'll likely remain breezy, as temperatures soar to 70°+ (current trends are for the low- to mid-70's!).  The record high for Philadelphia is 64°, and it's looking like this record will be shattered.  Below is the GFS MOS Data that shows modeled record temperatures for Thursday:

Christmas Day

  High pressure will continue to build in to provide for a dry and mild holiday, with temperatures still uncertain but likely nearing 60°, but it really depends on the timing of our frontal passage.  

  

Record Warmth This Weekend

    The past month and a half have certainly had plenty of warmth, as winter-haters rejoice and snow-lovers weep.  November was 5.6° above average in Philadelphia, and so far through December 10th, the city is currently 7.0° warmer than average.  Coincidentally, that number will increase through the weekend thanks to temperatures that will likely set records.  The record high for Saturday & Sunday in the Philadelphia area is 65°, with Monday's being 69°.

    Ahead of an approaching system from the west, a ridge will build in across the Northeast for the weekend, allowing for warm air and moisture to build into the region.  The process that will help cause these abnormal temperatures, called warm-air advection, is the transport of warmer air by the flow of the atmosphere.  In our region, this usually happens with a southerly flow, which brings warmth and moisture from the Gulf Coast, which is what we'll be seeing through Monday.

    For our Saturday, morning fog will give way to a mostly to partly sunny sky, with calm, dry conditions and a high in the mid-60's.  Sunday will then be much of the same with even warmer highs into the upper-60's, and some places could even threaten the 70°-mark!  Below is GFS data showing highs well into the 60's and approaching the 70's when our average high is only in the mid-40's.

GFS MOS data from Friday showing record warmth this weekend!

    However, all good things must come to an end, as clouds will increase on Monday with that cold front approaching from the west.  Though the region will continue to see highs well into the 60's, showers will be the story for the PM hours, with upward of a half an inch of rain possible at this time.  On top of the warm temperatures and rain threat, windy conditions will be in play due to the pressure difference between the strong low pressure over the Great Lakes and high pressure off the coast.  Breezy conditions will continue into a beautiful and slightly cooler Tuesday.  Below is the GFS (American) model's look at Monday's rainfall for Monday late afternoon/evening.

Friday afternoon's GFS model run showing Monday's rain threat.  Image courtesy of TropicalTidbits.com