Monthly Archives: June 2018

Updates

WEATHER UPDATE-SATURDAY AM

[su_note note_color=”#d9f2da”]Noon Update: My optimism about clearing after the warm front passage was apparently misplaced.  This was a “low-confidence forecast” for a reason.  The warm front hasn’t really made it past Philadelphia and it won’t until later in the afternoon.  A light drizzle has also developed. The GFS model was apparently correct.  Showers are beginning to develop to our south and may move in earlier than expected.  [/su_note]

[su_note note_color=”#d9f2da”]Yet Another Update 5pm: The FV3-GFS shows lower chance of thunderstorms this evening. The warm front never made it north of Philadelphia!  Poor model performance today![/su_note]

The showers have pretty much ended after 8 AM here, so yesterday’s NAM forecast has verified.

The question is – how much clearing  and brightening skies will we get?   The NBM (National Blend of Models) has things brightening considerably about 1 PM.

Current upper air analysis shows dry air punching in from the south (see 300 mb humidity graphic below)

But lower levels of the atmosphere are pretty saturated.  So I’m not sure if the drab, cloudy conditions will improve, but I think they may.

Current analysis shows much of the dynamics for thunderstorms remain to our south:

lifted index
Current radar and lifted index

Those dynamics supporting thunderstorms are expected to move northward by late afternoon giving us scattered thunderstorms.

So brightening skies (maybe) for much of Saturday with scattered showers and thunderstorms re-developing around 4-5 PM.

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST- FRI EARLY EDITION

I’ve been reviewing the latest runs of the NAM and GFS models.  Both models have a warm front moving up from the south and through Philadelphia  between 8 AM and 10 AM Saturday morning.

There are large differences between the models regarding the precipitation.  Specifically, the NAM cuts off the precipitation following the frontal passage, allowing things to dry out after 10AM until late afternoon, when additional thunderstorms may form.   QPF values are less than 0.30 inches water.

However, the GFS maintains enough dynamics for showers and thunderstorms throughout Saturday, especially early afternoon Saturday.   QPF values are over 1.20 inches water!

I’ve been looking for clues as to which model forecast is correct.  For some reason, the non-operational NBM and EKDMOS blended models have not been available for the past day. Other models (CMC, ICON) support the wetter GFS.

My own experience with warm fronts in summer is that things tend to clear out following a warm frontal passage.  So I’m inclined to go with a more optimistic forecast.

So… after early morning showers, expect precipitation to end by 10 AM with mostly cloudy conditions, but skies may brighten considerably.   Expect scattered thunderstorms to re-develop late afternoon into evening.  High 81.   (A lower confidence forecast than usual.)

For Sunday, a mix of sun and clouds.  Warm and humid.  High 88.  Chance of thunderstorms late afternoon.

I’ll update Friday evening, but it may be late.

[su_note note_color=”#d9f2da”]Fri 10:30 pm update: Tonight’s NAM continues with a break in the showers between 10 am and about 5pm Saturday.  So staying with the optimistic forecast.[/su_note]