Premium Weekly Outlook: More active, progressive pattern this week

Good morning and happy Monday! Overall it was a beautiful weekend, especially as lower humidity settled into the region on Sunday. But a more active, progressive weather pattern is expected this week. This comes as a strong heat ridge begins building over the Rockies and Western Plains this week and the AO/NAO remains positive with lack of high-latitude blocking. This pattern will allow some ridging to build into the region this week with heat and humidity building. Multiple disturbances are expected to move from the Great Lakes/Ohio Valley through the Northeast and Mid-Atlantic, with threats for showers and thunderstorms this week.

First off,  high pressure will be moving off the Mid-Atlantic coast, with a return southerly flow developing over the region today. Mostly sunny skies are expected this morning, then some clouds will be increasing and filtering the sunshine as a warm front approaches.  Humidity will begin increasing later this afternoon as dew points rise into 60s. More lift and instability ahead of warm front may lead to a few isolated showers or thunderstorms popping up over the region later this afternoon. Temperatures will rise into lower to middle 80s area wide. South-southwest winds today will allow for sea-breezes to keep coastal sections in the 70s much of this afternoon.

It will be partly to mostly cloudy for the evening and overnight period, as a warm front continues to lift northward through NJ, PA and NY. A mid-level shortwave associated with A decaying convective complex from the Midwest and Ohio Valley early today will also be moving through the region overnight into Tuesday. Some elevated instability with MUCAPE will be present as the night goes on, with isolated or widely scattered showers and thunderstorms developing around the region. Otherwise a warm and muggy night expected with temperatures not likely falling past the upper 60’s.

On Tuesday, more hot and humid conditions are expected with a mixture of sunshine and clouds, as some convective debris or high cloud cover lingers over the region. A southwesterly flow with 850mb temperatures around 17°to 19° C will likely support high temperatures at least in the upper 80s to lower 90s away from the coast. If there clouds clear for more sunshine some spots, especially from New York City southwest to Philadelphia, may even get into the middle 90s. Over much of Long Island and other south-facing shores, high temperatures in the upper 70s to middle 80s are more likely with more afternoon sea-breezes.

3km NAM model showing high temperatures in the lower 90s away from coast on Tuesday

3km NAM model showing high temperatures in the lower 90s away from coast on Tuesday

A cold front and mid-level shortwave approaching the region could trigger more scattered showers and thunderstorms during afternoon and evening hours on Tuesday. A lack of large scale forcing and more deep-layer shear appears problematic with 500mb winds fields somewhat weak and height falls minimal with these features. But with dewpoints in the upper 60s to lower 70s, instability will increasing. Some stronger 700-850mb wind fields will support 0-6km bulk or effective shear around 30-40kt. So there is chance for that some thunderstorms will get better organized with time and become strong to severe.

The main threat from stronger thunderstorms would be some gusty or isolated damaging wind gusts over 40mph with high DCAPE and steep low-level lapse rates. But low-level winds are also forecast to back along a boundary over Northern NJ and Lower Hudson Valley — supporting a few supercells that could produce large hail. Supercells may also produce an isolated, weak tornado with 0-1km shear between 15 to 20kts and low LCL heights. Thunderstorms with some heavy rainfall and localized flash flooding are also possible with precipitable water values around 1.50’ to 2.00”.

NAM showing high MLCAPE juxtaposed with 30-35kts of bulk shear on Tuesday afternoon

NAM model showing high MLCAPE juxtaposed with 30-35kts of bulk shear around the I-95 corridor on Tuesday afternoon

On Wednesday, high pressure from Southeast Canada will briefly nose into the Northeast and try to push the cold front slowly southward through the region. As it does so, humidity will be decreasing with a drier airmass behind it. But a few isolated showers or thunderstorms during afternoon and evening can’t be ruled out, especially south of NYC. Temperatures on Wednesday will be just a little cooler than on Tuesday with high temperature mid-upper 80s to near 90s with more west-northwest downsloping winds over the region.

An area of low pressure tracking through Great Lakes and Interior Northeast will cause the frontal boundary to stall over Northern Mid-Atlantic region and then return northward again by Wednesday night and Thursday, with heat and humidity increasing again. Temperatures may rise back into upper 80s to middle 90s over parts of the region. More instability and a couple more shortwaves may help trigger more scattered showers and thunderstorms over the region as the front lifts northward again by Thursday afternoon. Some isolated strong-severe thunderstorms will be possible again as more instability and shear will be present.

A more organized frontal system associated with a deeper shortwave trough from the Great Lakes will moving be through the Northeast and Mid-Atlantic regions by Thursday night and  Friday. It is too early to discuss a lot of details of this system, as there are some model variances or differences early this week. But most guidance indicates stronger wind fields and more sub-tropical moisture will be associated with this system. Some EML (elevated mixed-layer) advection ahead of this system with steeper mid-level lapse rates may enhance instability. These dynamics overall will likely lead to some higher risks for severe weather and/or heavy rainfall over Northeast and Mid-Atlantic regions.

6z GFS showing low pressure developing along frontal zone Thursday evening

GFS model showing low pressure developing along frontal zone Thursday evening

Some more pleasant, drier weather may occur behind this frontal system, at least for the first half of this upcoming weekend. But as the heat ridge amplifies in the Rockies/Plains,  a larger trough with lowering heights will start moving into the Northeast as the weekend goes on. As this occurs, another cold front is likely through some point late in the weekend next with perhaps more some showers and thunderstorms. Temperatures are likely to be near normal for this weekend. Then a cooler, drier airmass may arrives in by early next week.

Stay tuned more premium updates throughout this week, especially on severe weather and heavy rainfall. Forecast thoughts and details will likely be fine tuned as we get closer to each day.

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