Partment of Meteorology and Atmospheric Science at Penn State.

This is Weather World.

Good evening and welcome to Weather World.

I'm your host, Alison Sousa joined here with forecaster Rob Lydick.

Now Rob, we've enjoyed some summer like warmth here, but temperatures are going to take a tumble by the end of the weekend.

Yeah, big changes are on the way.

We had a little bit of moderation out there today.

I think we've got one more warm day before we step right back to spring on Sunday.

We'll hear more about it in Rob's short range forecast.

Also on tonight's show, we have Marissa Ferger with the extended and Karl Schneider with week two trends.

But first, here's Rob and thanks very much.

Alison out there today was very changeable, seeing some times of clouds and some times of sunshine.

This is a time lapse in Dubois.

Also seeing a few showers across northeastern portions of the Commonwealth.

I did want to point out the green up out there.

My allergies certainly detecting that green up.

We see all the green on some of the trees.

Dandelions here on this webcam.

If you have any yard work that you need to do, I think you should do that tonight or tomorrow because some big changes are on the way.

As we go to the second half of the weekend, here's a look at the satellite and radar picture.

You see some of those showers instability showers dissipating as we go through this afternoon and some clouds beginning to fade as well.

I think we'll see a quiet night tonight before things become a bit more exciting as a cold front approaches.

For the start of our weekend tomorrow.

We did have a bit of moderation today.

Temperatures did make it into the lower 80s in southeast Pennsylvania.

After back to back days of daily records in Philadelphia, they hit 91 degrees the past two days, the warmest such days in April since about 2009.

In Philadelphia, meanwhile, temperatures were in the 70s for most of us.

Upper 60s in Bradford Lake breeze kicked in and kept things on the cooler side in Erie today mid 50s for daytime highs there.

Compared to yesterday, most places anywhere from 7 to 15 degrees lower this time yesterday.

I think we'll see a warmer day tomorrow before we really take a step back on Sunday, you can see some places that have really taken a step back in the last 24 hours on the order of 43 or 44 degrees, a big change behind that boundary in portions of the northern Plains and Rockies, where now they are dealing with some snow, several inches of snow actually on the way for portions of the Rockies.

Here on the temperature map, we play the game, find the front a lot here on Weather World 53 in Omaha, 83 in Saint Louis.

Right there.

That battleground between the air masses is where we find our biggest threat for some severe weather today, some of that already lining up across the corn belt all the way down through Kansas, where they do have some tornado watches that have been hoisted.

We won't see a tornado threat so much here tomorrow, but I do think we have the chance for seeing some damaging wind gusts as this cold front begins to move on in.

Here's a look at our surface map for tonight.

Our storm system that was kind of pesky throughout the week continues to depart off to the east in its wake.

High pressure is in control though.

We will see a bit of an easterly component to the flow.

That will bring an increase in cloud cover in eastern Pennsylvania.

Tonight.

As we go into the day here on Saturday, cold front begins to approach.

Clouds begin to thicken up.

I think we will see the arrival of some showers and storms in western Pennsylvania as we go into the afternoon, holding off until the evening in the Central Mountains.

If you live in eastern areas, a mainly dry day, though, you may be socked in with clouds for a majority of the day before the showers and storms move in tomorrow night.

Some of those will linger in central and eastern Pennsylvania as we begin the day on Sunday.

Then eventually that cold front moves offshore.

We quiet things down in terms of precipitation, though a secondary boundary will also keep the chance for a few flakes.

I think in northwest portions of the Commonwealth with a frost or freeze likely as we begin the day on Monday.

So for tonight, mild and quiet temperatures remaining in the 50s, it will be a good night for taking a walk outside or maybe a campfire.

Enjoy it now while you can, because I think tomorrow night will be contending with those showers and storms ceasing sprinkles in northeast Pennsylvania this evening.

Winds become more easterly and that will bring in more clouds, I think, in eastern Pennsylvania as we go through tonight.

But these temperatures about 10 to 15 degrees above average for this time of April tomorrow.

We do have a slight risk for seeing some strong storms, primarily to the west of I 99.

The biggest threat will be for some damaging wind gusts, large hail and also some locally heavy rain.

I think a widespread half inch of rain is looking likely with this boundary as it moves through some portions of Pennsylvania as we go through tomorrow night.

But tomorrow itself, we'll see an increase in cloud cover in the east increasing ahead of that boundary.

Also in the west, with some gusty storms arriving during the afternoon holding off until the evening.

In the central Mountains.

Everyone sees widespread rain as we go through Saturday night, but tomorrow is going to be the last of the warm and muggy days because by Sunday.

Take a look at this.

We're flipping the script.

40s and 50s.

Morning rain tapers off during the afternoon in the east, some spotty drips and isolated flakes to the north and west, and a frost freeze likely by Sunday night.

Back in a moment with more.

And that trough you just saw in the break there is going to have a big impact, at least at the beginning of our extended forecast period.

You can see it here as it pushes into the Pacific Northwest, moving into the Intermountain West and now nudging towards the Plains.

We also had a weak trough move through the region last night, and that fired off some of the showers and thunderstorms as it moved through.

By the time we get to Monday, the trough that we were first talking about out to the west will actually be pushing into New England, and that's going to be ushering in some very cold air for us, especially compared to what we've experienced this week.

So a very chilly day in store for Monday, and then a very cold start to the Tuesday as this trough starts to lift out a dry day for Tuesday and then Wednesday, this disturbance to our north is actually going to have a weak cold front at the surface.

Push through.

That could fire off just a few scattered showers and then bring some cooler air in for Thursday.

Friday is the day I have the least confidence in.

While it looks like this big ridge of high pressure is going to be nudging into Pennsylvania, there is the chance that this upper level low offshore could actually bring some cooler air and more easterly flow into eastern Pennsylvania at the surface.

And if that's the case, we could have a cooler day there.

I'll talk about that more in a moment.

Now we're at the surface here talking about that precipitation tracker.

This is where the model is picking up some precip offshore.

There's a cold front offshore.

Here are the lines of equal pressure our isobars.

This is an area of high pressure here centered over the Great Lakes.

That's ushering in the cooler air to start the day on Monday.

So a chilly day in store there.

That high pressure is going to park itself over Pennsylvania to start the day on Tuesday.

And that's a recipe for a very cold morning with clear skies and light winds.

I think most places across the state will start the day below freezing.

So if you have any fruit trees, that could be problematic.

As this pushes eastward, we're going to get some return flow from the south and that will help warm things up on Wednesday.

But here's that weak front I was telling you about at the surface.

That may fire off just a few scattered showers.

Bring in some cooler air for Thursday, but not as cold as we're going to see earlier in the week.

And then as we go into Friday, this particular model is not showing too much in the way of problem weather.

But if these winds become more easterly, that's where we could have some cooler air and some more cloud cover across eastern Pennsylvania, and that could make temperatures a little bit cooler.

We're going to be a little bit optimistic though here, starting off the week with a very chilly day with temperatures at least 15 degrees below average on Monday with a few light rain and snow showers, we'll clear out late in the day, and that's going to set us up for a very cold start to the day on Tuesday.

Although it will be a bright day, and with the bright April sunshine, our temperatures will slowly rebound still below average for this time of the year.

Wednesday Earth Day ahead of the front, we warm up with temperatures still slightly below average.

Just a few isolated showers.

We cool off again behind that front on Thursday, but it should be a dry day.

And then Friday is our day of greatest uncertainty.

But I'm holding out hope that temperatures will be at or near average.

Up next, Karl Schneider will tell us about the week two period.

This week two period takes us to the end of April, and during this period, temperatures look to be roughly around average, maybe a little bit below average, with some chances of precip pretty seasonable for this time of year.

As always, we'll start the segment with a look at the average highs and lows during this week two period.

And you'll notice these numbers creeping up every week.

On this graphic we always show the average highs and lows.

But one interesting piece of information I'll give you is about the variance of the average high temperatures.

We're nearing the climatological maximum for that.

That is to say Climatologically.

It's very typical for us to be on the temperature roller coaster at this time of year.

Interestingly, during my period, temperatures look to be much closer to these values, maybe five degrees plus or minus through much of this period.

So temperatures a bit more normal, if you will, during this week two period.

We'll start, as always here with a look at the European model upper air pattern.

Starting off with today's flow pattern, I'm looking for two things ridges and troughs.

And also as these lines begin to separate, an indication of uncertainty in the forecast.

Right now we're seeing pretty big ridging across the east.

That's been the theme over the past several days.

But as you heard about in Marissa's extended forecast, that is going to change heading into next week as this trough propagates eastward by the middle of Marissa's period, Troughing will be commonplace across areas in New England and off the coast.

But I'm watching the development of a shortwave ridge to our west, which will move eastward by the end of the week, and by the time we start the period on Saturday of Week two.

Even though the magnitude of the shortwave ridge has decreased, we'll still be under the influence of southwesterly flow aloft, which I think Saturday will be on the milder side of average.

You'll also notice all the spaghetti becoming quite tangled around this time.

I'll switch modes here.

Jumping ahead to Monday of week two.

That feature will move off to the east and in its wake will be roughly in zonal flow.

You can see the black lines, which are the mean flow pattern, are roughly west to east and really an absence of color indicating the flow pattern isn't departing much from average.

That's an indication to me that temperatures will be roughly on either side of average during this period.

Also looks like a weak flow pattern.

And so when these weak flow patterns, it can be very difficult to time a specifically where these systems come through and kind of nail down where those rain chances are.

But moving into the middle of the week, notice that things start to change, maybe an indication that another cold front goes by around Wednesday of week two.

And in its wake, some cooler air filters in behind that, and the strength of that anomalous trough increases towards the end of the week.

But overall, we're still very zonal in terms of this flow pattern.

And so I think temperature extremes on really departing from either side of average are not very likely during this period.

So thus my confidence is pretty low.

So I think the warmest day of this period is Saturday at the beginning of this week two period.

After that, there will be a cold front that will go by around Sunday of week two.

That will bring a chance of showers in its wake.

Temperatures look to be roughly around average before we have the potential for another front to go through around Wednesday of week two.

But as I said, with these, the flow pattern being so weak, my confidence and timing out any of those systems is pretty low, so remaining rather unsettled.

But I think another big story here is that I don't think we'll have any large magnitudes of precipitation.

We're just generally dealing with cloudy conditions and showery conditions with temperatures roughly around average.

So that's week two trends.

We're back with a recap up next.

We'll have a mild and quiet night across the Commonwealth, with temperatures remaining in the 50s in most locations, 10 to 15 degrees above average for this time of year.

More clouds rolling in in the east.

I think the clouds will hang tough there through most of the day on Saturday.

Elsewhere, they'll be thickening from west into the central mountains as we go into the afternoon, and some gusty storms arrive ahead of a cold front.

Those will also produce some downpours through the central mountains in the evening.

That cold front continues to cross as we go through Saturday night, and that will bring rain eventually to eastern areas.

A soaking rain that will be beneficial, especially off to the south and east.

So tomorrow, the last of these warm days, some places in the lower 80s southwest.

Before we really flip the script on Sunday, temperatures only in the 40s and 50s.

Morning rain begins to taper off during the afternoon.

Much cooler and breezy as well.

Some gusts to 30 miles an hour.

A bit more sunshine in the west as we go into the afternoon with a few spotty drips and isolated flakes.

Even northwest, a few frost and freeze conditions likely to begin the day.

Some big changes on the way.

Alison, including for you.

This is your last show here.

What's next for you?

Yes.

So I'll be graduating in May, but I will be pursuing a master's and PhD at Colorado State University, and my research will be focusing on convection in Africa.

That's awesome.

We really appreciate everything you've done for us behind the scenes, helping with editing weather wise.

You've been a director, also a host.

I think you've been a host now for at least a year.

So this is just really great to see you excel and exceed.

You're on to great things in Colorado.

We hope you come back and visit us.

I will definitely it's been a pleasure to be on your screens and have a great weekend.