Welcome to the weather blog - a regular Monday through Friday discussion of the weather! While the discussions usually will only come on days I'm working, I'll issue special updates when the weather warrants. I will always post to let you know when no discussion is expected if I'm away on vacation, etc. - if no update is here and no info is available, that likely means the server has temporarily gone on the fritz and I will update as soon as technically possible. You'll find a quick weather synopsis linked to the daily forecast at the top of the page, a general non-technical weather summary below, and when available (most days) a detailed technical meteorological discussion will follow by mid-afternoon. My email is contact@mattnoyes.net. This blog is for you, so I hope you enjoy it! -Matt Noyes
General Weather Summary:
Weekend Pattern and Expected Precipitation thru Sun. AM:
A very energetic and moisture loaded scenario will unfold across New England on Friday, bringing very strong thunderstorms and torrential locally flooding rains.
The airmass clash set for today was already unfolding early Friday morning as a cool front settles southward into New England and woke White Mountain residents early with a strong cluster of thunderstorms. Thereafter, multiple areas of thunder developed across both Northern and Southern New England - the former a result of the cold front and the latter a result of warm and humid air still surging into Southern areas, further fueling the upcoming duel of airmasses.
With so much tropical air in place across Central and Southern New England, a period of sunshine to further heat the land and the air, the slow moving cold front settling only gradually southward and eventually stalling over Central New England, and an energetic disturbance moving through aloft, Central and Southern New England are primed for strong Friday afternoon storms. Given the amount of available moisture in the atmosphere, any and all of these storms should contain very heavy rain that will both reduce driving visibility to near zero, and produce ponding of water on roadways for street flooding and hydroplaning concerns. Frequent cloud to ground lightning strikes will come with most storms, damaging winds exceeding 55 mph will be found in the stronger storms resulting in damage to tree limbs and power lines for some communities, and the core of heavier storms will bring hailstones that may exceed three quarters of an inch in diameter. You can follow these storms through radar links to the right of this discussion through the day, and check for active warnings using the link under "Active Advisories and Current Conditions" to the left.
Friday afternoon's storms will simply grow more numerous and expand in coverage late in the day until they combine into a swath of rain through most of Central and Southern New England Friday evening, and this rain will persist - heavy at times with embedded thunder - through the overnight. By Saturday morning, the cold front dropping south through New England will have stalled out across the region as a stationary front, separating drier and more comfortable air seeping southward across the Canadian border from the deep tropical airmass across Southern New England. A wave of low pressure will ripple toward New England along the frontal boundary Saturday, and this will focus heavy rains through the first half of the day. The heaviest rainfall amounts, as indicated in the map above, will fall near the boundary between airmasses, across Central and Southern New England, and flooding of streets, streams and low-lying areas is likely where the heaviest rain falls. Later Saturday afternoon, as this wave of low pressure moves through, a break in the action is likely, which doesn't mean we'll be entirely dry, but means we're likely to see a more scattered nature to showers and downpours. Across the far north and farther south on Nantucket, rain will be much lighter with a few breaks of sunshine possible on the island.
The problem with this setup is that this first wave of low pressure and the associated slug of rain is far from the end of the weather story. As we've been looking at together in these discussions over the course of the week, there's a series of circumstances that will continue to favor heavy rainfall at times through the upcoming week. The first part of the puzzle is a jet stream that features a dip, or "trough", over the Ohio Valley, to our west. The jet stream is the fast river of air that steers our storm systems and acts as a thermostat for the atmosphere - separating cool and dry air to the north from warmer air to the south. With the jet stream dipping over the Ohio Valley, this turns winds aloft out of the southwest - blowing from the Gulf of Mexico into New England - with energetic disturbances caught in the jet stream winds moving over New England and serving to focus this moisture. At the same time, a large area of deep high pressure will strengthen over the Western Atlantic. While high pressure usually favors fair weather, this fair weather center will be too far east for us, keeping the quieter weather of the ocean, and the clockwise flow of air around this high pressure center will usher a south wind directly out of the Bahamas, up the East Coast and into New England. Not only is this a moist direction, contributing additional moisture to the Gulf tropical moisture already en route, but remember that together we've also been watching an area of thunderstorms north of the Bahamas that will become caught in this southerly wind flow.
Though the National Hurricane Center has put Hurricane Hunter aircraft on standby in case the need to investigate this system arises, no immediate development is expected as of this Friday morning post. Nonetheless, this cluster of thunderstorms represents additional deep tropical moisture that will be available for new slugs of heavy rain over the next several days as it interacts with the stubborn frontal boundary across New England. Our interest in the tropics won't end there (read on...) but this moisture will gradually feed up the East Coast and the east side of the Appalachian Mountain Chain, interact with both the Gulf Moisture and the nearly stationary frontal boundary to wring out another slug of rainfall that will likely recoil from south to north later Sunday. By Monday, the high pressure center over the Western Atlantic - full of tropical air - will build westward just enough to give us a break in the action with tropical air aloft early, and this may result in a few breaks of sunshine, but the moisture streaming north from the Bahamas will likely arrive as another slug of concentrated rainfall later Monday.
This pattern will linger through the week, though it's quite possible that the midweek will bring another extension of the Western Atlantic high pressure cell, meaning some sunshine would be possible each day, leading to afternoon tropical downpours. But there's a wild card between the weekend and the midweek that we need to pay careful attention to, and that's the indications I'm seeing that another disturbance will move into the waters north of the Bahamas late this weekend, and this will have the potential to spin up into a Tropical Depression or even a Tropical Storm late this weekend or early next week. With a deep southerly flow up the coastline and into New England, we'll need to watch this carefully to see if it does indeed develop, as this pattern of deep tropical flow means that New England and the East Coast serves as a magnet for tropical moisture and disturbances.
The bottom line is that folks should have basement pumps at the ready, and communities across Southern New England should prepare to deal with flooding of low lying areas and along small rivers as soon as this evening and tonight in the case of urban and stream flooding, and by later this weekend elsewhere. We'll obviously need to watch rivers carefully, as well, though there will be plenty of breaks between heavy rain swaths, which will allow for runoff and may keep flooding limited to smaller rivers, though if a tropical system does indeed develop and head north early next week, that would raise the stakes on mainstems, as well.
Stay dry and stay safe! Enjoy your weekend.
Matt
Matt's Technical Meteorological Discussion: Updated Friday, June 23 at 3:30 PM
Short term: Morning convection sparked by two forcing mechanisms - North Country by cold front and Southern NewEng by warm and moist surge which allowed sun to come out and further increase instability. This has resulted in two areas that will be the focus for convective initiation Fri afternoon - the front which is now becoming stationary, and an outflow boundary laid almost perfectly along the MA Turnpike including line from ORH to BOS. It's likely this outflow boundary that will serve as the pathway to strongest cells - already a cloud street has been laid out with W to E oriented cumulus line along the boundary and as vorticity max approaches this afternoon, this should capitalize on CAPE values in excess of 2500 J/kg across Southern NewEng. But on top of this we're seeing 3 km EHI values south of the outflow boundary that are somewhat concerning - around and over 2 in SE MA/South Shore - along with steep 0-3 km lapse rates, both of which gives concerns for supercell development in this area. LIs run -5 to -6 along and S of outflow boundary, and my thinking on this is that as convection develops along both the stationary front and outflow boundary, new outflow boundaries will result in new convergence, and instability (esp the steep low level lapse rates and otherwise light wind flow) will allow for each of these convergence points to act as convective initiation hotspots and that means numerous thunderstorms filling in and most of them with plenty to work with to become severe. Overall, however, while damage will be done in Central and Southern NewEng, the biggest threat is likely to come from torrential rain that will blind travelers, cause street and urban flooding along with hydroplaning concerns, and as larger convective clusters establish, flash flooding will be a concern.
That flash flooding concern continues thru the overnight as numerous thunderstorms should gradually congeal into one shield of rain with embedded thunder. While guidance indicates a few inches of rain with this shield, it's important to realize that the thunderstorms Fri Eve and early Fri Ngt will be so moisture loaded they'll drop a few inches of rain on their own! Shouldn't be all that tough to achieve my forecasted amounts.
Going to have to run but want to let you know my thoughts on early next week. For the past few days, models have been handing off the baton for which one carries the feature but consistently have been showing vortex development in the area where the wave is currently north of the Bahamas. The trick here is that it's not the current wave, which still is having trouble lining the mid level center up with several low level vorticies that are evident on visible sat imagery. Instead, this lead wave is still expected to serve as moisture contribution to our late weekend/Monday rains, though the trend for Monday is to absorb this rain farther west across PA and NY. I would expect at least the swath of warm and moist advection sweeping north associated with this moisture influx and its associated wave to bring a swath of rain into NewEng and esp Western NewEng later Mon. While the closer we get to midweek the more we get into the Western periphery of the Western Atlantic ridge with deep tropical air that should allow for some sun and tropical downpours, we need to watch very carefully the next tropical wave mentioned above as it comes north of the Bahamas. The GFS is currently carrying the baton on spinning up a low level vortex and bringing it up the coastline...given the fact that this solution has been touted by at least some guidance products for several days for late this weekend, carrying the system north during the first half of the upcoming week, I think we have to acknowledge this as a real possibility and pay very close attention this weekend.
If I have time I'll post something over the weekend....see you here Monday regardless.
Matt
