* Cool, Comfortable End to Week *

10:00am Thursday, July 24:
After hitting 91 degrees on Wednesday (only the 7th 90-degree day of the year), a strong cold front moved through with some strong storms Wednesday evening. As usual, rainfall varied widely with over 2 inches reported in some spots (mainly south of Route 30) while less than 0.75" fell in some parts of NorLanCo. Official rainfall at MU was 1.54". This front will slowly sag southeastward today and skies will gradually clear this afternoon and evening as high pressure builds in from Canada.

The next 48 hours will be another remarkably comfortable spell with moderate temperatures and dew points more in line with the opening days of June than the closing days of July. Last Thursday and Friday (July 17 and 18th) brought us a similar air mass with highs of only 80 degrees and overnight lows of 58 and 55, respectively. Last Friday morning, the low temp in Bradford (PA) was 41 degrees! Today through Saturday morning will be a repeat performance with highs today and Friday only near 80 and overnight lows back down in the 50s--open your windows and give the AC a rest! Many cool spots in northern PA will again drop into the 40s tonight and Bradford has a shot of reaching the upper 30s late tonight!

A return of southwesterly flow will bring some clouds and somewhat warmer and more humid conditions Saturday afternoon into Sunday. A warm front and upper-air system may combine to trigger some showers or t-storms (severe?) on Sunday, before yet another cold front moves through with showers on Monday. A deep jet stream trough will then take hold for much of next week--the result will be a frequent clouds and a few showers, but a continued lack of extreme heat and humidity. Therefore, it's hard to see how we'll hit 90 degrees again this month...and so we'll likely end July with just FIVE 90-degrees days (7 total for the year). In case you are wondering, five 90-degree days in July would be the fewest in five years. Here is a list of the # of 90-degrees July days the past five years: 2013 - 10; 2012 - 12; 2011 - 14; 2010 - 18; 2009 - 1. FYI, the historic average # is 10 and the record # is 21 day in July 1921.

11:00am Monday, July 21:
I'm back from my annual family vacation to the Rockies, and it's always a bit of a shock to my system returning to the mid-Summer humidity of the East. Having lived the past month in low dew point air of the Rockies (dew points in the 30s and 40s), returning to dew points in the 60s and 70s (the stuff of PA summers) equates to a doubling to tripling of water vapor content in the air...and the "sticky" feel to the air that I personally despise.

Fortunately, the air temperature is only seasonably warm for my return to the Commonwealth (with highs in the 80s), and a quick glance at MU weather records reveal that I missed only four 90-degree days in the past four weeks. I do see several spells of very muggy conditions (dew points in the 70s) in the past few weeks, but having only four 90-degrees--and a warmest day of the summer (so far) of just 94 degrees--is NOT an extreme summer for Lancaster. If fact, the six total 90-degrees so far this year is less than half of the normal amount (through July 20). While not nearly as anomalous as our near-record-cold winter, this summer is following the "no sustained heat" storyline that I forecast at the start of June.

Looking ahead, I see more of the same with an active jet stream bringing another upper-level trough and front through the region on Thursday. Prior to this front, the heat and humidity will build a bit each day with highs approaching 90 degrees on Wednesday. While I can't rule out an isolated shower today or Tuesday, the best chance for showers and a scattered t-storm or two is Wednesday afternoon and Thursday as the trough and front slowly cross the region. Clouds and a shower may linger into early Friday, before somewhat cooler/drier air filters in later Friday into Saturday.

Assuming that the trough/front exit on schedule (it's not unusual for such a front to get hung up along the coast this time of year), high pressure will make for a nice, comfortably spell Friday into Saturday. Another jet stream disturbance--perhaps a large trough--then looks to arrive here Sunday into Monday, so another cloudy, unsettled spell is likely...and it could last into the middle of next week.

Issued: 7-24-14 (Horst)